Well I'm sure this isn't a complete list, but I just bought a '90 2wd wagon and it is going to need struts ($500-$1000 depending on where you get them done). I have heard that the Automatic Transmissions on the '90 and '91's may be touchy and they are big bucks to replace or repair (I've heard $1500-2500). Not sure how you check them out though other than listening for slippage or sluggish shifts (especially between 2nd and 3rd gears). If the one you are looking at is an LS, then the Air Suspension is a LOT more than the struts I mentioned above. I would do a quick visual on the CV joints too to see if they were torn. There are a number of seals (cam seals and valve covers) that typically leak on these cars, but I 've heard they aren't too bad to get repaired. No idea on the cross member bolt repair, but that sounds like it may have taken some damage from below. Crossmember supports the engine and transmission in the front. YOu would have to have a mechanic check it out. Never heard of it in the "standard" Legacy issues. You may want to do some reading on the Ultimate Subaru Message Board
It's a very good and thorough board. As to price, doesn't sound too bad if it doesn't have any major issues.