Check the oil seperator hoses laying on the top, back of the intake manifold. Use a 5mm or 3/16 allen wrench to remove the engine cover (4 each 1/4 turn bolts) and you will see the two hoses. Good chance they will be shot. Look for obvious rips or tears and give the hoses a squeeze. If they feel like play-doh, then they are for sure shot. Common problem, but easy to fix. Parts = $70-80, can be replaced in about 30-45 miniutes with a screwdriver and a pair of pliers. Good negotiating tool to get seller to drop price a few hundered bucks.
And for sure check outer front CV boots.

I bought my '01 with 70k miles, super clean , maint records, sport, cold weather, nav, brand new tires and brakes, everything but PDC, for $21k a few months ago, but I had to look long and hard to find it then negotiate for a week to get the price down. I too did not want to pony up $30k for the newer X5 at this time. Outer front CV boots were ripped at time of purchase, and I got the seller to drop $150 due to that, but in retrospect, now that I have changed them (myself, about 8 hours total labor and $70 in parts for all four in the front), that wasn't enough money, should have gotten $400-500 off.

Hope this helps. Good luck.