Can you post pics of what you got?
Its a very good idea to rebuild the truck. If it feels like it is well used, i recommend doing a diff rebuild with fresh balls as well as rebuilding your shocks with a rebuild it. Make sure your CVDs also run true and remove any stripped or starting to strip screws. Verify all bearings spin freely, replace those that don't.
In general when I buy used I expect to spend about $10 - $20 more bucks replacing worn stuff.
I would read up on your charger and make sure it can charge nimh packs.
The best money you probably can spend would be on a good charger like the Reedy Quasar Pro. used they go for a good price and have discharge capability too. For a discharge tray, just pickup the Indi Octane 2. Can be had for around 20 bucks these days and works well. Its what I use, I have this combo of charger and discharger and it really keeps your batteries running well.
For motor you probably need to see what class you are going into and find out if its legal. If not, you will want to buy a class specific motor.
As for the servo, I would look into buying a metal gear servo at the minimum. I think the 645MG would be a good choice and can be had for a fair price new on ebay too. You can also try the Z590M servo. They should be sufficient and are all over the place cheap.
Good luck and feel free to ask anymore questions.