Good for you rebuilding the old KDX. I wish I still had my '84 200.
Where are you located?
Since it is so expensive to ship forks off for service and you are not doing it yourself, you will probably want a reputable local dealer, repair shop or suspension expert to do the work.
Perfomance wise, the weak links on the old KDXs were the forks and the brakes. Not much you can do with the stock brakes. A swap to an '86 - '88 front end would help alot and get you a disc brake.
As for cheap fork fixes, Kawi usually made the fork springs too soft and did not add enough fork oil. You can cut a few coils off your stock springs and put in enough oil so that the level is about 4" from the top with the springs out and the forks compressed. This will help a bunch.
Adding Race Tech Cartridge Fork Emulators with the stiffer springs would make a big improvement, and if there is a Race Tech authorized shop in your area, they could give you a quote. Probably will cost alot, but would make an improvement.