[Eliot Ness]

Yeah Kirk, if you're going to delay the purchase of variac at least build a simple and inexpensive DIM BULB tester:

http://www.antiqueradio.org/dimbulb.htm

That might save you from frying a power transformer due to bad or incorrectly installed capacitors or other components.

An isolation transformer is not a bad idea either, especially if you start working on sets without a power transformer. If you can find a good buy on a BK Precision 1655 or a Sencore PR57 these combine a variac and isolation transformer into one unit (with handy built-in metering).