I have welded aluminum before with a 300 amp Miller wirefeed welder before, but technically, you can go with a much smaller unit as long as you are not welding thick material. that being said, I have found it is much easier to weld aluminum with a wirefeed welder if you put at least twice as much time into prep work.
The biggest factor that I have found in welding aluminum with wire is to clear the oxide layer off as much as possible and to always keep a decent speed pushing the bead. You can't use a pull technique for welding aluminum with wire unless you want to destroy tips every time you start. 100% argon is also necessary (well, technically, you can use an argon/helium mix too). I have found that I generally have to turn up the argon higher than if I were doing stainless steel or mild steel.
Another huge factor with welding aluminum is knowing how soft the filler wire is. It is much easier to use a spool gun, but if you have to use the internal feeder then the hose has to be as straight as possible and the rollers adjusted properly. Also, contamination is a huge issue. Make sure that the liner is clean of any non-aluminum burrs and that everything is as clean as possible. If you followed all of the above then you should be able to weld at least lighter gauge aluminum.