Hey, welcome to the club...
I just had my LIM gaskets replaced a few weeks ago. I've owned my NA 3800 (2001 SE) for almost 3 years now. I wouldn't shy away from any 3800 powered car because of the problem. I would buy my car all over again, if I had to.
You won't have to worry about the UIM having problems, as with the SSEi'* the composite UIM (the problem part) is replaced with the supercharger.
My car had 67,000 miles on it when mine went. So, if you go by mileage the 48k one probably is fine for now, and the 85k will probably need it soon if not done already. I'd say your chances are a lot better that the work was already done on the 85k one, though.
I had my LIM gaskets done by the dealer. It is a pretty substantial bill if you have them do it. I would guess your bill would be around $800-ish if you don't do it yourself. Mine was about $700 for the labor, and the new aluminum gaskets are around $60.
Hope that helps your decision... Good luck with your car hunt either way.