Boy oh boy....many questions, so I'll try to answer them in order.....
1. A TIG and Stick power source is whats known as a "constant current" power source while MIG is a "constant voltage" power source. They are fundamentally different and are not directly interchangeable. You CAN get a wire feeder for a constant current source but it is known as a "voltage sensing" feeder and generally not cheap.
2. A "transformer" is a basic buzzbox (AC only) a "rectifier" is basically a "transformer" than can also put out dc (some are dc only while some can be selected) An inverter, in my experience, is a newer technology that has to do with the way the current is generated. It leads to all the newer, lighter, smotther, more powerful welders that you see today. I have used an inverter based machine that was constant current/constant voltage selectable, allowing MIG/TIG/ and stick welding from the same machine. The Miller website has much useful information about these above topics..
3. A welder must be equipped with the proper connections for a foot pedal to be able to vary the output remotely. (usually an amphenol-type plug) and is it doubtful that any older machines would have it (over 15-20 years) You COULD just get a TIG torch and hook it to your Stick welder and it WOULD work(with the proper shielding gas) but there are some difficulties with this
1. no ability to control your current output
2. most welders that are made for TIG have a high-frequency circuit for initializing your arc. You would be limited to a scratch-start TIG setup which can be very frustrating to the new user (lots of wasted tungsten, stuck electrodes and less hair on your head)
3. (if you do try this, be sure to reverse your cables so that the TIG torch is NEG and your ground is POS; AKA "Straight Polarity" which is a must for TIG welding ferrous materials)
4. Any suggestion on a particular welder is basically, in the end, a) what do you REALLY need, and b) what can you REALLY afford. Answer these questions truthfully and go with that. Stay with the big names and I doubt you'll be disappointed. Buy a little more welder than you decide on if you are serious about taking on big projects, just enough if you're at your limit now.
5. An aluminum TIG setup differs from a basic TIG system in that it MUST be AC capable and it has a constant high-freq circuit that always runs (this HF stabilizes the arc and helps to break up the refractory oxide on aluminum) rather than just at arc initialization (as on steel) MOST AC TIG outfits are backward compatible to be able to weld Steel and almost every other material under the sun. Again a welder that is this capable is also good for basic stick welding, and if found at a good price, indispensable.
6. An aluminum MIG setup MUST have a pull type gun rather than the feeder due to the difficulty in pushing the soft wire (think pushing a rope) It must be pulled from the gun end. Also, to add aluminum capability to most basic MIG systems you must buy the converter module that turns the DC current (which 99% of MIG systems are) into AC. You cannot just hook up the gun and go (there are a few welders that have it built in, but they are meant for just that purpose) I just bought an Alum. MIG setup for a Miller at the shop and it cost me $520 CDN (the welder had the module already built in; not sure on the cost of it otherwise)
7. Another drawback to welding aluminum on an already undersized machine is that aluminum, contrary to common sense, take substantially MORE current to weld than, say, mild steel; due to its huge thermal conduction. If you have trouble welding 1/4" steel, you'll be lucky to properly weld 1/8" aluminum with the same machine.
8. The TIG gas is obviously important, and I have had great results with a 75% Helium/25% Argon mix on ALL materials. The helium lets you "cheat" a bit on alum/magnesium as it helps to inject a little more heat into your weld, letting you weld thicker materials longer. It isn't cheap, bit IMO, its worth every penny
9. And finally, yes the TIG torch is the same, but the type of tungsten changes with steel and aluminum. With steel, you'll want a RED (2% Thoriated) ground to a point 2-3 times the electrode diameter while with aluminum you'll want a GREEN (pure tungsten) or BROWN (zircon.) which both are left very blunt and will round when welding due to the AC current. I have welded Alum. with the RED with good results but the Green/Brown don't do well with the DC current (they have lower melting points)
I hope the above information helps in some ways. Not to bring you down, but your weldpak probably isn't worth much used, and $500 and change probably won't get you everything you want. It will cost you just as much to get an older machine to do what a newer one will cost off the shelf. The Miller EcontoTigs are a good machine, and worth the $1500 you'd pay for it. I guess the next step is to take the above point #4. What is "the next step" worth to you?
Good luck, and I hope I cleared a few things up for you.
