While I also would be a little put off by the claim of a "rebuilt suspension and trans", I have been following the 500E threads for about 2 years on this forum. From what I understand, the engine is pretty much a stock MB 500 v8, with lots of stock parts, unlike some of the later AMG cars. I believe several previous posts indicated that they weren't really more expensive to maintain than other MB v8s, with the plus of having a w124 body essentially hand assembled for sporting intent.
If I recall, the main thing people said to avoid was a car that had front end damage involving the sub frame and suspension. Also, an inexpensive part that needs upgrade involved a power steering hose. The last thing that comes to mind is radiator issues, as the cars are supposedly prone to running hot. Try a search on 500E, and a post in the hot rod forum down the page. There are some clean ones out there, but I have noticed the price going up over the last year in our local papers.
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