strantor,
About making your own welder- like you I have considered this, especially in the days when cash was tight. But the position is that you would have one hell of a job developing building and testing a rig with the facilities and performance of the sort of machine that we would both like to own. Yes it would be possible, but, I hate to say this, impracticable.
What might be worth considering is adding an AC inverter, to allow aluminum welding, to a basic high current MMA or TIG rig. The best source of components would be faulty rigs which you can get quite cheaply.
Even so, I think we would be better off just biting the bullet and buying new or second hand. You could soon recoup the cash if necessary by doing a few welding jobs here and there. When I had access to a MIG welder it was surprising how many people wanted the odd bit of welding done here and there- often repairs to garden gates.
spec