The R129 Mercedes-Benz SL320, The Last Inline-Six SL, Is Perfect For Budget Buyers

Cost-of-living pressures are no joke right now, and for car enthusiasts that sting cuts a little deeper. Spending money on a fun set of wheels is one of the most discretionary things you can do, especially when the price of everyday life keeps climbing.

The good news is that classic cars open up a world of opportunity for buyers on a budget, as there are still many hidden gems that you can buy for the equivalent of a deposit on the average new vehicle. These are the kinds of machines that still make you glance back after you park and start walking away, all while delivering the kind of analog driving experience that modern cars often struggle to match. A prime example is the fourth-generation R129 Mercedes-Benz SL-Class, which blends old-school vibes with timeless roadster appeal.

The Last SL With An Inline-Six

Key Facts To Know

• Sold in the US from the 1994 to 1997 model years
• Powered by the M104 engine with a 3.2-liter displacement
• Rarer than the V8 version of the R129 generation SL

The SL-Class is a legend, not just among Mercedes-Benzes, but in the wider automotive world, too. The SL nameplate, an abbreviation for the German translation of “sport light,” was born from racing, specifically with the original 300 SL “Gullwing” launched in 1954 as a road-going counterpart to the W194 race car, which marked Mercedes’ return to motorsports after the Second World War.

The production version owes much to influential New York importer Max Hoffman, who convinced Mercedes that a high-performance sports car aimed at the booming postwar US market would find eager buyers. Hoffman also persuaded Porsche to build the 356 Speedster for America.

The R129 SL-Class debuted at the 1989 Geneva Motor Show, leaving behind the curvier, more classic lines of its predecessors in favor of a clean, straight-edged design crafted under the Bruno Sacco era. Beneath its angular skin, the R129 was a showcase of cutting-edge Mercedes engineering. It introduced an automatically deploying roll bar, integrated three-point seatbelts, adaptive damping, and a surprisingly aerodynamic shape for a roadster of its size and shape.

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The R129 was sold in the US from 1990 to 2002, and the base SL320 trim, offered from 1994 to 1997, tends to be the most affordable option today, together with the SL300 trim it replaced. Its 3.2-liter inline-six is generally more reliable than the V8 and V12 engines offered in this generation of the SL, and it carries historical significance: the SL320 was the last SL-class to feature an inline-six, continuing a tradition that dates back to the original 300 SL.

Of course, you sacrifice some performance with the smaller engine. The SL320’s 3.2-liter inline-six produces 228 horsepower, compared with 322 hp in the SL500’s 5.0-liter V8 and 389 hp in the flagship SL600’s 6.0-liter V12. The SL320 replaced the SL300 in 1994, which also used an inline-six and offered similar horsepower to the SL320 but lower torque.

In the US, the R129 was generally paired with a four-speed automatic, though early SL300s briefly offered a five-speed manual. The SL320, however, came exclusively with a five-speed automatic.

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The SL320 used an engine known as the M104. It’s a smooth, naturally aspirated engine with dual overhead cams and 24 valves, though fuel economy wasn’t all that better than in the V8-powered SL500, which is part of the reason the M104 was discontinued.

After the engine finally bowed out from the Mercedes lineup around 1999, the automaker wouldn’t return to the inline-six format for almost two decades. Inside, the SL320 and other trims in the R129 generation, the cabin blends classic Mercedes luxury with driver-focused ergonomics: leather seats, power-adjustable controls, a wood-trimmed dash, and a functional, uncluttered layout that felt modern back then and reassuringly analog today.

On the road, the SL320’s inline-six should be more than enough for everyday driving. However, excitement only starts higher in the rev range, with the redline coming at 6,200 rpm. The benefit is that you get to work the engine a lot more compared to the V8 and V12 models, which build speed without being pushed hard. Buyers will find the ride definitely more on the grand tourer side than that of a sports car.

How Much To Pay Today

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Over the past five years, the R129 SL-Class has quietly shifted from “old car your grandpa might drive” to a legitimate collector’s car. That transition isn’t finished yet, which means there’s still time to get in before prices climb further. Scan today’s listings and you’ll find SL320 examples starting at around $10,000. That still sounds like a bargain, but it’s worth noting that about five years ago, that was roughly the average asking price.

At that entry point you’re typically looking at cars with well over 100,000 miles today, though that’s hardly a deal-breaker if the maintenance history checks out. Drop below the six-figure mileage mark and prices begin to climb quickly. Clean cars in the 50,000-70,000-mile range often push past $20,000. Step up to the V8-powered Mercedes-Benz SL500 and you’ll usually pay only a modest premium, while the V12 Mercedes-Benz SL600 is rarer and can command roughly double the price of the lesser variants.

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When it comes to reliability, the SL320 is often considered the safest bet in the lineup. Its inline-six has fewer moving parts than the V8 and V12 options, which keeps complexity down and long-term durability up.

The engine was also widely used across Mercedes models of the era, so parts availability and mechanic familiarity remain strong today. The biggest issues tend to not be so major, typically things like leaks from the valve cover gasket, front timing cover, and oil filter housing. As with any classic car, a solid maintenance history is key, and having a trusted mechanic familiar with older Mercedes models will definitely make ownership less of a headache.

Inline-Six Comeback In Full Swing

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As mentioned above, Mercedes waited nearly two decades after the M104’s retirement to bring back the inline-six, this time in turbocharged form with the M256 engine, introduced in 2017. Available in 2.5- and 3.0-liter displacements, it debuted in the Mercedes-Benz S-Class and has since spread across much of the lineup, though not the SL. The M256 features lightweight aluminum construction and 48-volt mild-hybrid technology, which in some variants powers an electric supercharger for quicker spool and smoother response.

Despite the ongoing push toward electrification, Mercedes, much like several other automakers, shows no sign of abandoning the inline-six again. AMG has developed a high-output version that produces 429 hp on its own, while in some applications it’s paired with plug-in hybrid systems. The most potent is in the 2026 E53 Hybrid, which delivers a launch-control-ready 604 hp, roughly matching the power of the outgoing E63’s twin-turbo V8.

Mercedes-Benz

The inline-six is set to appear in more performance applications at AMG soon. The Affalterbach tuner has confirmed it for a new C53, which will replace the current unloved AMG C63 and its plug-in hybrid powertrain built around an inline-four. It remains unclear whether the C53 will use a plug-in hybrid system like the E53 Hybrid or rely solely on the M256 engine alone, as in the AMG CLE 53, where the engine is rated at 443 hp. We should know more soon, however, as the C53 is expected to debut for the 2027 model year as part of an updated C-Class range.

A Timeless Mercedes You Can Still Afford

Mercedes-Benz

The beauty of the R129 SL320 is that it still delivers the things enthusiasts crave: long-hood proportions, a silky inline-six, and that vault-solid Mercedes feel. Yet it costs less than many new-car down payments. In a world of ballooning prices, it’s a rare reminder that a car you can genuinely desire doesn’t have to come with a huge price tag.

Sources: Mercedes-Benz