I seem to be hearing alot of the E38 needing work, the 1998 i had in the UK with full service History i didnt think was bad, granted i spent about 1.5-2k in 3 years.
Controls arms went (once every 8 years), few bushes, in know the seals on the vacuum things in the back of the V12 where on the way out (no problem, just about going). That was it.
Other than some really minor pipes and clips.... I think it has to do with how it was maintained, however the one i had (pic below) had a service all the time, and didnt show any problems.
I would be looking to run the E38 for 5-8 years till i got the new 2009+ F01 (i think it is) second hand.
That said i prefer the 750iL design, but i wont move on the engine as i like the V12 for power and running, (doesnt drink much once at speed with the cruise on).
If you want to keep the E38 then pay it, if your not sure then sell it and move on. However i do miss mine as enjoyed it alot, but i couldnt import it with me so had to be sold (even though i want a 99+ now due to toys installed i would have still liked the 98 i had as it almost everything and was VERY nice to drive).