first, are you sure you want to tackle tig welding aluminum on that frame, on your first attempt at this? don't get me wrong.....i believe in the can do spirit and all of that sort of thing but, "tig"ing aluminum is NOTHING like welding steel. especially thin sheet aluminum. even if you are successful at welding it. you will need to work it back to being straight again. are you sure you really want to tig this frame?
even a cheap tig that can do A/C welding is not going to be "cheap". the really cheap tigs out there only do d/c. that's why they are cheap. i also don't recommend it be a scratch type tig. you'll need a foot pedal control, not a preset or finger tip control. the mini torch head you want will be an added cost too. then there's the tungsten electrodes, filler rod, gas and personal protective gear. i don't know what you're willing to spend on this so, i thought i'd let you know this ain't gonna be cheap or fast. the learning curve for welding aluminum is pretty slow. i've known guys who have welded all their lives who haven't mastered the skill of "tig"ing aluminum. if you're sure about the cost thing and the dedication. i'll be happy to walk you through it.
please, don't think i'm giving you a hassle or hard time on this. if you truly want to do this, i don't mind helping. i just don't want you to get all gung-ho and rush into this un-informed only to find out how much it REALLY takes to tig weld aluminum. on the other hand. the satifaction level is OFF THE CHARTS!!!