k, could i solder aluminum??? or do i have to weld that as well...
Yes, you can solder aluminum, as well as any non-ferrous metal, but again, it requires a little more heat than a propane torch will give you. If you run that torch on mapp gas, you're more likely to solder aluminum successfully.
There is a product called Durafix, which allows you to solder aluminum and create a joint that's equal in strength to the aluminum you're soldering, so it's suitable for a go-kart *if* the aluminum you're soldering is strong enough. Thin aluminum isn't very strong.
While a lot of the guys here like to tease me about my frequent recommendation of the durafix soldering rods, I've found them very useful for many things and not just soldering together projects. I've fixed radiators, locks, BBQ stands, engine parts including heads and blocks, transmission cases, etc.
However, wire welding for steel is a lot easier and more forgiving, though the machine will cost you a couple of hundred dollars for something useful as compared to the many imported $129.95 and less units that have a ridiculous duty cycle. The duty cycle is important because a 10% duty cycle means you can weld for a minute then stop for 10 minutes. When you first start welding that's actually okay because you're going to stop and inspect what you're doing constantly while trying to get the hang of it. But after about 10 hours of laying beads, you're going to find that low of a duty cycle annoying at best and you'll be looking for a better, higher power machine.
You might also consider used welders. Ebay and craig's list are littered with them and sometimes you can get a really good deal.