Definitely take a class or get some instruction from someone who knows welding. You'll get a chance to run some good quality equipment that is properly set up so you actually learn to weld instead of fight with equipment. Feedback on what's going right or wrong from someone who knows will get you going faster. You can't buy a cheap machine that will do everything. Forget about any serious aluminum welding with a cheap MIG welder. You might get lucky and run an occasional bead by using a teflon liner and keeping the hose very straight but usually you'll spend more time clearing birdnests from the drive rolls than you do welding. I have a spool gun for my 250 amp L-Tec MIG machine which works really well for aluminum but cost $800 by itself. If you want/need to weld thin aluminum you'll need a TIG capable machine. That's generally going to mean a different power source from your MIG with high frequency, a different torch, regulator, foot pedal to control amperage etc. You can find deals on used stuff if you are patient enough. I scrounged together a real nice TIG/stick welder for about $600 but it took several years to score all the components (Miller Dialarc HF300 amp, Miller foot pedal, 200 amp air cooled torch, regulator). It's a sweet machine that can stick weld almost anything but requires a 240V 90amp supply. If you are going to weld at home check and see what you have available for power to supply your machine.
Steve