To treat rust none of the above you need to bare metal or cut it out to treat rust, but preventing then any wax oil will help, I find schultz to be best in my opinion. But a good coating of por15 then a waxoil will give you have a dam good protection.
Agree with micra biker. Its about the low amps you pay high money to buy high amp welders to weld thick metal. For car repair work the panels are thin so require low amps I got my 2nd hand clarke for under 100 and is ideal for diy home stuff.
Sounds a fair bit for spray cans it can be done and like magpie says prep is everything. But one offer is http://board.moparts.org/ubbthreads/showflat.php?Cat=0&Number=2331682&page=0&fpart=1&vc=1 the classic 50 dollar paint job. It does work I have done it; if u have the time u can get the finish.
How much do u mean by extensive; as a panel can be done but colour matching and blending will be possible issues. Halfords cans can be alright and not too bad price wise. You need to be more specific of the size and finish your after as large and perfect will be hard to achieve.