First off mate, dont waste a cent upgrading/putting more parts on your iron horse, that would be a pointless waste of money, ride it until it breaks or throw it in the bin.
Second off, DONT build your own bike, if you cant spend 4 0r 5 hundred on an AM set up you cant afford to build a bike, building anything half decent will cost you well in excess of 1 or 2 thousand.
Heres you only option, for 4 or 5 hundred bucks you cant buy anything new, certainly not a semi decent AM bike, you will have to start saving, wait until you have saved as much as you can, certainly more that 4 or 500 bucks if you want a 2nd hand AM bike, if you save 1k you will be able to get a decent 2nd hand bike, maybe you will be able to find one for less if theres no rear suspension.
OR
Id be looking out for something like a giant STP dirt jump bike, 2nd hand you can buy one for 500 bucks, they are bomb proof, you can jump all day long on them, any type of DJ (dirt jump) bike would be sweet for you, and they are real fun to ride, like a big BMX (26'' wheel size)
Youve written this in the 29 section, if your looking to have fun, jump things, a bit of free ride, you dont want a 29, you want a 26 or at most a 27.5.
So go look for a DJ bike, you will have tonnes of fun, they are bomb proof, you can be hucking 10'' drops if you want, and they are cheap 2nd hand, just make sure you get the right size.
cheers