If your just wanting to get out on the track and have a go, all you will need is the rims, slicks, catch cans and axle and peg sliders. This will get you on the track, going lap after lap with no worries. No, you wont be up the front battling it out with ringo on KTM, but it will get you going at minimum cost.
If.......... you want to start racing harder with the boys and girls near the front, then start upgrading things like ignitions, rotors, brakes, suspension, bars, seats, pegs, gearing, valves, cams, pistons, carby, airfilter, plastics, exhaust. But just remember, if you suck as a rider none of the mods above will make any difference, as all the races are won into, through and out of the corners, but first you need to aquire the skill to be able to put the bike on the edge under brakes and learn where the edge of grip is so you can turn the bike to its maximum potential.