you can get them pretty cheap these days,
I just bought a 200A AC/DC TIGMaster for $1,000 off ebay for work.
which is a good commercial welder, being AC to do aluminium as well. does HF / Ramping / Pulse all the good stuff.
i think they sell a decent DC version which will be cheap, Not sure if you have an ARC welder but most ARC's can be converted into a TIG with the simple purchase of a Tig handpiece, Regulator and Argon bottle...
as for tigging a Coupling too a SS pot, amps wouldn't push much past 90A or you will just burn the pot if its a keg with 3mm walls, you might push upto 110A
Have you done much Tig welding?? practice a lot first on the thickness of materials your using, thin wall pots burn away pretty fast, I'd suggest not having the hole any bigger than the coupling, might be eaiser to tig it onto the vessel then put the hole through it.
The other option would be to take it too a small metal fabricator, prob find it would be done as a cash job and wouldnt charge you much.... i used to do all that kind of stuff when i worked for a small business, now im @ a big one.. we have a min $80 charge out fee....