[Arran]

You can get a fairly serviceable digital multimeter from fleabay or from an outfit like Harbor Freight, for under $30, the ones I get typically come from a Canadian Tire Store, on sale for under $20. Signal generators are a dime a dozen, they turn up at ham fests, flea markets, craigslist, and even at garage sales, the thing is that they usually need a rebuild by now much like the radios. Unless one is a modern lab grade unit I would not pay more then $15 for one, most were given to me by people who were junking out. Vacuum tube voltohm meter are useful, those can usually be found in the same places as the signal generators, same with most test equipment.
Tube testers turn up fairly regularly but fleabay is not the place to get one. Emission testers will not tell you everything but decent ones will tell you is a tube is a dud or shorted. For some reason the audiophools believe that you need a mutual conductance tester but those are really of minimal value for audio tubes. You don't need an isolation transformer for an AC set with a power transformer, maybe AC/DC radios if you intend to work on enough of them, I don't but some do. You already have a variac, or an autotransformer as you called it, those are a very useful thing to have for slowly powering up a radio.
Units like this G.E combo unit are the types I usually avoid, especially if they are incomplete. They are truly hogs for floor space, the general layouts are pretty much alike regardless of brand, and the record players usually need an overhaul which can get expensive. Because of this they aren't really in high demand, even the better units. You could just right this off to experience, or post an ad on the alternative forum looking for a junker for parts.
I hate the so called "repurposing" butchery a lot of morons pull today, usually very badly, but I think that a combo unit with a junk or missing changer is a good candidate to convert into a bar radio. Take the changer drawer out, and built a new one with holes for glassware and bottles (if there is enough room) to go where the changer used to live.
Regards
Arran