The parts are readily available most anywhere. The rear end will hold 600+ hp easily. I don't know anyone with real problems using the stock 10 bolt with a good posi/limited slip differential added to it. There are plenty of guys running turbos pushing over 500 hp without issue. You can add a billet cover to beef it up a little if it makes you feel better. Go for GM gears if you can afford them. Otherwise, Richmond will do the trick. Gears and an install kit should be just at $300. A posi/LSD will run anywhere from $300-$700 depending on what you buy.