A 1990 300E has the potential of being the best care you have ever owned. Like any othe make, it also could be a disaster waiting for your wallet to open. The only way you will know which one of the above is waiting for you is by doing three things:
1) Do a lot of research on any car model you want to buy. Search the archives for words such as 'buying', 'purchase', '300E', 'w124', especially the longer threads. 10 or 12 pages of what to look for on this car are in the archives. Mostly that earlier engines had weak valve guides that were upgraded, and may need replacing to the upgraded guides by 100k miles on the older ones. Also that the AC evaporator is expensive to repair when it goes out. Finally, MBs are built like tanks, but fall apart fairly easily if they haven't had regular service by the book - insist on complete records. Facts will make you a better deal.
2) Pay for a Pre Purchase inspection (PPI) from a good independent 'MB only' tech. One can probably be found in your area under the 'Good Shops' forum on this site. Also pay to have a good body shop check it out for signs of collision damage and re-painting.
3) Compare as many of the 'suggested purchase price' (Kelly, Edmunds, etc) to come up with a price for your area. Subtract the laundry list of repairs the PPI. Factor out another 5-10% for surprises. If you and the seller agree on the resulting price, you bought a car. If not, you avoided some repair costs.
Good luck on the search - these are great cars, but like anything else ever made by man, are not perfect.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E