I believe all MB's are good quality. (We are not comparing KIA, Lada & Tata).
Even after Chrysler was part of the group, the 129 was still prodused at MB in Germany and not in the US, if that's what you are all worrried about.

Being a car prodused from 1989 till 2002, it is now considered an "old" car. However, at the value you can get one these days, I believe they are VERY good value.
All cars, including KIA, that are between 6 - 20 years old will have some issues.

I would go for the latest model year SL500 that you can afford, and forget about the older engine with 20 hk more power, as I do not think you will notice.
Anyway, they are all top of the line MB's, and as such, repairs might not be cheap.

Good luck, Per.