Car upgrades ( engine, transmission, rear end

What class do you plan to race it in? Once you've decided that find out what is the minimum requirement for that class. Don't buy anything. Just figure out what that will cost. Then start combing Craigslist, Kijiiji etc. for a used engine, transmission and a posi rear end. Don't forget about suspension mods and some decent tires. Add up all those prices, and 50% for all the stuff you haven't thought of and see if its a number you can live with. If it is, take a $100 bill and flush it down the toilet. If you can do that without flinching then you may be up to the task. Good luck and have fun.