I would recommend getting a cheap MIG not an arc. Arc's are usually used for structural applications. With a MIG you can do structural stuff (with a high $$ one) as well as intricate type stuff.
you cannot weld aluminum with an arc welder. You can with a mig or tig. Migs that are setup for aluminum are usually pretty expensive, but they are usually a dual spool so you can weld alum and steel both. If you aren't a good welder I wouldn't even try aluminum, it's a whole different animal than steel. If you plan on welding a lot of stuff and a lot of different materials, get a TIG. you can weld anything with a tig, you just need a variety of electrodes and a variety of rods, and you're set. TIG's are expensive though, I think the cheapest one that has an AC setting (needed for aluminum) is about 5 grand (Miller). If you need advice on techniques for different materials, ask me, I have welded just about every metal possible.
As for cutting, I don't know about the rest of you guys, but I am surgical with a cutoff wheel. Get a triangle square, and some stabillos. If you intend on doing a lot of steel cutting get a chop saw with a Grinder type blade for steel.
As for bending, don't even try to bend tubing yourself, it's a pita, Get a tubing bender from Harbor Freight or something. You can get them cheap. I have tried 4 or 5 different types of DIY benders that I spent hours building, all to fail. You might be able to bend 1" dia or smaller tubing using a large dia pipe as a die and a whole bunch of grunt. It all depends on what you want the bend for... If you don't care if the pipe gets indented (structural type) apply heat, if you do care (flow type, ie exhaust pipes etc) you need to get a good quality bender to not crush the tubing.