On your find, The KX fork "upgrade" is only an upgrade if the fork is from a 2002 up. And rarely, if ever, is the stock KX fork spring rate going to co inside with the stock KDX shock spring rate, nor will the valving at either end. You're going to have to find a suspension guy that really knows his shit to balance that thing out.
Also, you may be getting snookered on the OS tank, the stocker holds almost 3 gallons. IMS does make one that holds 3.4 and comes with it's own shrouds, here's a link to a pic of it
http://www.imsproducts.com/Backgrounds/110shadow.pngMy buddy's KDX220 will get about 80 miles on the stock tank, and that's at race pace.
It sounds like a good find, just make sure you get the stock parts with it in the deal.