Tepid performance by today's standards? What standards are you looking at

6.3: 0-60 in 6.5 seconds
6.9: 0-60 in ~7 seconds

Plus an ungoverned top speed of probably ~140MPH on both models.
Compared to much newer MBs with V8s and V12s, they might be slower; but compared to even a 560SEL (~7.5s) they're much more powerful and faster. As for money pits, it depends what type of car you buy.

If you spend $20k on an m100 vehicle, EVERY rubber piece should be at MOST 6 years old. Every maintinence receipt should be in the glovebox. The interior should have no rips, the body absolutely NO rust, and the engine should run perfectly. Not a showcar - those are much higher - but a very nice example. The $3000 ones are the moneypits, with all 4 corners of the suspension needing major repair, the engine needing at least a timing chain, body repair and/or paint, etc. Stay away from the $3000 ones if you'e afraid of doing all this yourself - you WILL have to.

That having been said, ENGINE-SPECIFIC parts are VERY expensive. Water pump? At least $500. Injection pump? Buy a new car.