start out with a mongoose
as you can tell from my username i love mongoose. im not a fan of todays mongooses cuz they not as good as the old ones. i got a 1985 racing mongoose and i love it. if u get a walmart brand get a mongoose that meets ur demands. they are good quality bikes still its just the style is a lil off from the older ones. they will last for the time you need to get good at what u want to do and they hold pretty good off dirt jumps. after ur finished with it you can always keep it as a spare if ur new bike has given out on ya and is in the shop. this way you can still practice. as for upgrades, they are somewhat subject to upgrades but i would just get a whole new bike. good luck to ya.
e-mail me if u got any other ?'s about bmx