You mentioned it had some engine work with the 325e
There are some mixed opinions about that
You'd best be reading up on http://www.strictlyeta.net/
Basicly he took an ETA and stroked it out. Its basicly a modded "i" engine now.
If what you said was true then the previouse owner did the opposite as that guy from the site did and he started with the I motor and modded it
Hope the car is reliable and healthy. If it is then 4,000 is a good price for a clean IS!!!