So far some of those things that you mentioned are typical maintenance, ie, tires, breaks, exhaust (isn't SS) or did you run over something? I'd listen to these guys on this thread and / or have a mechanic check it out. It could be something not too too major, i.e., fuel pump. Only a couple more hundred bucks and you're in business.
I would have it checked out and would not lose faith yet. I always felt that the 190 wasn't as durable as the larger models, but I do know a lot of people who have success stories as well.
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)