The difference between a tig welder for steel and aluminium is quite substantial - so is the price. For steel it is just an inverter with rectified DC and a scratch start circuit, for aluminium there is extra stuff for 'AC'. From what I understand, the extras invert the voltage every so often to puncture to aluminium oxide layer. Then you have the 'pulse' features to regulate the amount of energy put into the weld. I spied a cheap inverter welder in paris earlier this year at Castco for about $149 euro. It was good for about 100A at 20% duty cycle. I normally use a more expensive inverter for DC tig and stick that is rated at 120A 100% duty cycle - but it is more like $800 euro in comparison.

It doesn't sound like an easy project to me to add the extras for aluminium welding - best be saving your money to buy the real thing, or get someone with experience to do the welding for you - aluminium is tricky and requires some training and lots of practice!