I'd ditto the above mentioned as far as buying a brand name machine. However when it comes to buying the gas bottle if you can upsize it (refering to the small portable bottles). I have a **** Clark mig welder which runs on 110v (20a). I've done all kinds of projects with it and it works good up...
I've done many head gaskets on motors with more than 100k and never touched a valve. It's possible to repair by just changing the gasket (that may not be the "correct" thing to do) but if all your going to do is keep the car a little longer there's no need to rebuild the head. It's really a case...