Get the vin, go to BMW ask to see if there are any comments and see what work has been done under motor plan.
Make sure the car has everything specd that you want like xenons, sportpack etc. As upgrading after is going to cost a lot.
And other than that just the usual exterior panels lining up and paint work etc.
My last '06 320d was on 145 000kms with no problems.
There's a forum member selling his 320d look here:
Link.
There was also a 135kw engine introduced into the prelci mid '07 so if you're getting a prelci I'd look for one of those. Type the vin into realoem to see which engine it has.