I can't imagine why tappets would get noisy at a specific mileage. Sounds like an old wives tale to me. My '88 300E has 168K miles and everything is still 100% solid. If you don't want noisy lifters, change oil hot and change oil often.
I've never heard of any Benz wheel bearings being a problem. The only acchilles heel I know of with these cars is the climate control system, I read that somewhere and doubted until a few months ago I had to put a pushbutton unit in.
Taken care of properly, these are solid, great cars.
Good luck with it and enjoy,
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in