Above all look for a history as said above. The ideal situation is to have one with a new Alusil block fitted, but the fact is most Nikasil blocks in the UK are fine. Mine has 123,000 miles and is totally smooth.
The other are that can cause concern is the auto box. Make sure it does what it should on a test drive and get used to the adaptive functions, such as changing down for you going down steep hills, and if you back off suddenly in a corner at high revs it will hold that gear until the road straightens again to stop you exiting the road backwards. Functional, but seems a bit weird at first.
Other than that it is usual E34 stuff, such as suspension bushes etc.
I thoroughly recommend them and I generally get about 20 mpg out of mine, and up to 28 average on a motorway at a constant 80 ish.