Don't just assume your old suspension is good enough to reuse. Look carefully at its bushings and all of its pivots. Make sure it will hold up over the years you plan to operate your glider. If it isn't up to the task, you're better off installing a new suspension as part of the glider process.
Regarding any component you're reusing, it's better to do the necessary maintenance on it when it is outside the truck. Spruce it up before you put it back in. It's best to have an engine rebuilt before you install it in your glider. That way, you know it stands a good chance of lasting at least as long as the frame.
Unless you have an electronics expert on your staff, you should seek technical assistance from your glider kit manufacturer when you're ready to install the electronics and wiring. This can be tricky, and since the smooth operation of your truck depends on it, you don't want to leave anything to chance.
Reseal everything that needs it, the drum drives, the pinions, the axles, and the brakes, when you have these components out of the old truck.
Call your glider kit manufacturer for technical support if you have any questions.