Sorry to hear you've been scared off. As Designo says, web sites where people look for help are full of people looking for help! I work in marketing and can tell you that satisfied customers almost never make a peep in public forums; dissatisfied people make a lot of noise -- so it can be very difficult to judge overall reliability from how many problems are reported.
Many people on this site love modifying their cars and spend a lot of time and attention on them; some, like me, never think about their cars unless they want to go somewhere. In which case I jump in the car and go. I get it serviced when it tells me, I get the official test done when they tell me and that's about it. I've only ever called the service out when a fool (um, me) left the light on and drained the battery.
My experience (a pre-facelift 2000 SLK230 and a 2002 SLK32 AMG) was that the facelift helped a lot, but that basically they're very good. My present one has 105,000 miles and works splendidly; when I get it serviced almost nothing needs doing. For what it's worth, it's my only car and I even use it for IKEA and similar tasks. I like it because I feel safe in it, it drives like a dream, doesn't need a lot of attention, and the roof down is a simple undiluted joy. That mix, together, makes it a great car.
In the end, you have to feel good about your choices, but don't be swayed by a long list of problems in a place where people list problems. You'll find the same for most cars or any kind of complex machine. If you're concerned about it, I'd read the reviews from firms like JD Power, who list the R171 as 5/5 stars for "Overall Reliability" (though in the end they prefer the Z4
) -- why not dig out the reports for the models and years you're interested in. I believe though that the individual car's history and build is vastly more important than the fault averages for the model -- the averages have to be pretty low, but some individual cars are frankly terrible. So whatever car you end up with, do pay very close attention to the individual one you buy.
Best of luck in whatever you choose, and have fun.