They are GREAT cars. I've had 3 all bought used, one used up but then fine after deferred maint addressed. The 735 is far less worry than a 750 and is almost as nice.
Same weaknesses as E34 pretty much plus they tend to go through driveshafts. The m30 is a great motor too.
5700 sounds steep for 15 yrs/145k but WTF
If it's a good example he will love it!