98 Custom Pro
None if you don't mind using more air
Best upgrade for 98, no need for batteries and with a double trigger you can have fun with the available 17 bps. My hopper holds 330+ balls which lasts you a game or longer. You can also buy offset adapters. Going to get lightning rod and vortex to get 30 bps later.
Buy Compressed air and a bigger hopper, You are set for a killer gun.