I'll second some of the posters who said the worst reliability problems generally came after the 1999 merger with Chrysler. In addition to some questionable parts-sharing, it was a HUGE distraction to management as well.
But that's an extremely generalized view, and there have always been model-specific reliability issues. For example the early W202 C-class had biodegradeable wiring harnesses in the engine compartment, and those have caused myriad difficult-to-diagnose problems. That was solved in '96 or '97. (Don't quote me). After that, the W202 became one of MBZ's most reliable cars. I have a '99 C280 which has been the soul of reliability, though I'm currently experiencing one niggling problem that I believe is electrical. I've also got an '01 SLK230 which has been extremely reliable.
If you want an MBZ, I'd focus on the older models. But that's no substitute for doing model-specific research here and elsewhere. You'd also do well to look at the annual Consumer Reports Buying Guides (a small paperback book) and their annual auto issue. While I'd be quite suspicious of other models from those years, I've heard that the 05-06 E320 CDI is very reliable. But the 07's and on switched to Bluetec, which came with a host of problems.
So I'd suggest you figure out what you like, then do some model-specific research. There are some gems out there that get thrown under the unreliability bus because folks don't do model-specific analysis. Also, if you can't do your own wrenching, you should reconsider this whole project, or learn how. MBZ parts aren't cheap, and labor is ususally 2/3rds of a mechanic's bill, often more. Buy parts wisely. I use AutohausAZ.com and Amazon.com and I tend to buy name-brand parts. Never buy generic electrical parts! As for features, I'd avoid cars with self-leveling suspensions, tons of wear, etc. Your best bet is an old, low-mileage car that was owned by someone who took care of it. And buy one from a non-snowy clime. Rust is the most expensive thing to fix on any car.
In 2017 I was able to buy a '99 C280 in California with 73K miles. After I bought it, I did full fluid changes (oil, transmission, brake lines, coolant, and power steering), and I ended up having to replace the MAF. Since then, I've replaced the battery, and done oil changes. After 30K more miles, the car has simply rocked. Yeah, it's a somewhat staid sedan, but it's a good stealth vehicle with decent performance (I got the Sport Package), and a great ride. My SLK is a lot of fun too, and clean cars can be had for $4-$6-ish if you shop well.
Best of luck, and I hope you get something you really like.
Which is not a great place to buy an old car.