What do I need to look for in a 98 E430?
A/C - make sure it was routinely serviced. it's a known problem and a costly one.
Transmission - if the car has over 50-60K, it should have had the transmission fluid/filter changed. although MB says it's fluid for life, almost every mechanic I know says it should be changed. this is also a regular problem (transmission probs at high miles)
engine oil leaks
oil change interval - if it's a 98+, is has the FSS system which indicates oil changes at every 10K miles. this is also a point of debate but there were lawsuits going around due to this. Ideally, you'd like to find an E where the oil was changed more regularly (always synthetic).
dashboard cracks
driver/passenger seat noise "clicks" when steering/shifting weight
front coil spring/strut cap support - there was a TSB on this. improper sealing caused some to rust through.
There are probably more problems that can be considered somewhat systematic, however, they are great cars. If the one you are looking at has complete and regular maintenance records, you're probably ok. I would also invest the small dollars for a pre purchase mechanical inspection from a Mercedes certified tech.
Good Luck!



