You should consider renting one first.
Welding aluminum is very different from steel, so you may not know your priorities until tried. For one, I think you will want water cooling for 1/4", and lots of current. If price is a concern, look for an old industrial unit. They have now come down in size/weight quite considerably, with many automatic features, and many businesses want these new units (and can afford them). So if you have space, you may get a good one for a fair price.