Don't worry about miles.. the 4.6 32 valve engine will handle many miles. No mater what you are going to have some parts wear out. As a car ages so do many parts.
Good new is the engine will last. (OK many have seen this many times before) I have 3 Mark VIII's all daily drives. Miles 95 - 150,000 miles, 94 - 140,000 miles, and my 93 210,000 miles. All run great.
Parts that wear out are usual wear and tear parts. Good news is if you are not afraid to turn a wrench a site like LvC will make the rears cheap and easier.
Look for the typical car issues oil leaking, tranny slipping etc....
When purchasing if you happen to know where one is drive by it a few times. Best time is in the morning and see if the car is sitting low. If so it has air ride issues. Depending upon what you want to do this can be a simple $400 fix to replace the air ride with conventional springs, or you can replace the components of the air ride system that need to be replaced (this will also cost you $150 to $400 depending on what is bad.
The best think you have going for you already is you have found LvC and have a connection to many Mark VIII enthusiasts who have experienced most of the problems that can go wrong with a Mark VIII. This experience will help you diagnose your issues, and help save you a lot of money taking the car to be serviced.
When I purchase my first Mark VIII and Lincoln in June of 2004 I was a take the car to the shop kind of person, a real gear head idiot (no comments from the peanut gallery). With the help of LvC and a few friends I have tackled some repairs that I never dreamed I'd be doing. What I have found is that the Mark VIII is a very easy car to work on and that the Members of LvC really know what they are talking about. What's fun is if I have a problem with a car I now will not look like the idiot on the side of the road with the hood up looking at the engine thinking "Yup that's the engine, what the phuck am I looking at".
LvC has turned me into a Mark VIII-oholic, I can't stop buying them and I love working on them.
Good Luck and I hope you find what you are looking for.
Personally I would look into a Gen 1 - 1993 to 1996 to start with they will run you a few grand less. Once you feel you really like the Mark VIII you can always move up to a Gen II with less miles and a few more options, heated seats, HID head lights....etc... 1996 LSC models also have HID head lights. Head lights are also an expensive issue but I won't go into that.