you can get a ton of spare parts "kits" check out
Fulton Armory. Bolt Repair Kit 40.00
Gas Cylinder Repair Kit, includes New USGI Gas Piston, Spindle Valve, Pin & Spring, and Gas Cylinder Shim Set 35.00
Spring Kit, USGI, all the springs for the M14/M1A Includes op rod spring, spindle valve spring, extractor spring & plunger, ejector spring & plunger, safety spring, hammer spring, and mag catch spring. 20.00
Trigger Group Repair Kit, includes Hammer & Trigger Pins, Hammer, Trigger & Sear and Hammer Spring Housing 40.00
Pin Kit, all the roll pins for the M14/M1A 6.00
what do you really need? that's a loaded question. are you a survival nut? do you think you will have to do your own gunsmithing? if so i would get every repair kit and all the tools i could. if you plan to shoot the gun say 500-1000 rounds over the course of it's life i wouldn't get anything. if the gun breaks send it into Springfield or Fulton armory and let a gun smith fix it. if you plan to shoot competitively and want to get into beding your stock etc i would get a pin and spring kit. if it had a factory glass bed job i would not remove the action from the stock. I feel you need to send it back in and have it reworked after removing it two or three times.
good luck. what brand did you get?