if you can justify spending the money on a decent AC/DC tig and having argon gas then go for it, but otherwise Id stick to MIG. Theres nothing wrong with welding steel with MIG it just wont look quite as pretty as a tig weld, but it all depends on how much value to place on some bling. A tig would not be suitable for your front end loader.
XCEL make some good Migs, I opted for their mig 165, which has been a wee beauty, but for what you plan on doing I think it might be best to go the next one up with a little more snot, the 200. Not for the overall grunt cos I think the 165 has plenty but for the extra duty cycle, also has digital settings rather than analogue dials.
My 165 cost a grand, then a gas bottle was about $280. CO2, costs 60 to refill and lasts bloody ages. For your alloy stuff you could always find someone with am AC tig and give them a box of beer for some welding, but that depends on how much you will be doing.