[morzh]

Roy

What makes a good Variac...

Well, my opinion: it should handle any radio that you restore without overloading itself. The output V-rangle of 0-150VAC is sufficient.
I have a 500 VA (read- Watts, though VA and a Watt is not exactly the same) Variac.
There is no consumer radio that this is not sufficient for.
And it is rather small.

(I also have a second one, 3kVA, up to 220V in and about 300V out. Used it once. But not too much money, so a good thing to have just in case. Don't think you need it at this point.)

An isolation transformer. Mine is 250W or so. Again, sufficient for most or all radios. Mine is a hospital grade Fluke, bought cheaply on eBay used equipment sale.

Now, about Variacs with isolation inside: due to size being an issue, the output transformer might be undersized, so you have see if the ratings are what you need. They might be sufficient for some (or even most) jobs but could be under-powered for some others.
I prefer things separate.