NEMA34 might well be overkill, given that a plasma torch doesn't weight much, and it won't be meeting any physical resistance.
Would 60V supplies work for you? They should be sufficient for the motor drivers, adn they're far easier to fidn than 72V supplies are.
As for machined ball screws, I have had no real problems with Chinese-supplied ones from AliExpress - sure, it's far from being a 100% consistent source, but for the money (and provided you don't go for the cheapest of the cheap) it's hard to go far wrong.