2001 was the first year of the "angel eye" headlights. Steering wheel and computer/navagation screen was improved for 2001. There are probably some other subtle differences I'm not aware of.
When looking at these cars determine if you like the fold down rear seat or not. The seats are different. I like the look better of the regular seat, but really like the fold down feature.(I actually carry things in my car)
Regarding interior color options, I don't know if the seats ever came two tone with red. I've only seen the blue or gray colors.
Regarding interior trim, I suggest aginst the simulated aluminum trim. It is a paint like coating that easily scratches and then looks terrible. At the time I was looking, 2 dealers told me the two tone seats only come with the aluminum trim option as part of a sport package. However, since having bought mine I've seen 2 cars with the two tone seats with the wood trim interior.
Lastly, you said with wholesale prices now around $30K.... For what kind of car?
Any M5, even a 2000, with a wholesale price around $30k would have to have a history. Meaning a repaired crash ,abused or extremely high mile car. My advice, don't look for the cheapest one you can find. Spend a few more dollars and get one in the best condition you can find for a resonable price
Hope this helps.