Ok, while Mercedes diesels are legendary, this model is ill advised. Here's why:
1st: What Thom says is entirely correct. The 3 liter engines are great. The 3.5 liter engines were simply bored out and failed more often than any other MB diesel I know of. Definitely NOT of typical quality and a scary gamble.
2nd: The W140 chassis are electrically complicated -- moreso than you want unless you (a) have a fat wallet or (b) enjoy electronics personally. Beyond that, they were susceptible to front end issues due to size and weight.
3. Importation. Nothing mechanically wrong here, but depending on your state/DMV...
4. For the collector, this is an uncommon vehicle, but not especially desirable. The W140 chassis' were considered very poorly styled and unless it's a 600SEC / 600SEL, buy it and keep it. Don't expect to turn it for profit.
DSLPWR