[Nekasrof]

-Royal Purple Racing 5w-20 (synthetic oil)I don't use it but I hear it's a good oil
-tail-shaft seal on the trans replaced at the same time( it started to leak due to sitting)Not necessarily a bad thing. Mine was leaking for a while and the seal is now replaced. It's a cheap part from Ford.
-drained and filled tranny last yearThat's a good thing.
-rear end diff, and axles upgraded to 31 splineVERY good thing.
-welded the axle tubes of the rear endDone for drags. Not necessarily a bad thing.
- has run it down the track a total of nine times. Not something that can be proven unless you know the car. I'd discount that number.

Check this site for what they are selling for/sold for and compare. If you don't know what you're looking at, I would get someone to look at it that does. One of my biggest pet peeves is brake fluid. What color is it? Is it clear? If not, why? The Ford dealer will charge around $125 or so to flully flush and bleed, including the ABS. It's one of the most overlooked maintenance items on cars.
After that, there are a host of things to look at. If you are somewhat mechanically enclined, you can do a lot of small stuff to keep that car in great shape for years. Ford made an incredible product. I've been racing mine for years. Bought it new. Has just under 104k on it. Engine and tranny haven't been rebuilt and it still puts down 306hp/330tq. Not that dyno numbers really mean a lot if you can't drive but all I know is I'm getting faster and faster in the car.
Anyway, it really is a great car even it it's stock condition. Good luck.