"Do I need to be concerned if a unit uses Toko or Coilcraft? Presumably if the theremin still functions, the risk of failure is just a normal burden of ownership."
That's a valid point as long as you can get some use out of the working theremin and not have to deal with fixing it right away. Chances are that sooner or later you will also need to tune one of those coils. If you use the right plastic tool it's usually safe but read elsewhere in the recent posts and you'll see someone that broke the ferrite core using a metal tool, and now it's a coil that need to be replaced. I see what you're saying but find out what model it is first and then if it's an older one you'll want a bargain price.
"https://instrumentalparts.com/keyboard-parts/moog-spare-parts/moog-etherwave-theremins-standard-spare-parts/ shows that the PCB costs $96 -- this seems to be the main component. (One could almost build one from the parts listed on the web site!)"
If they still have them that's worth considering, although they may not be in stock long. And you're right, that is the main component. The rest is just wires and potentiometers. Moog used to sell these for under $70 but they no longer offer them. I bought one board from the place you mention and later wised up and got the rest directly from Moog. If the theremin is cheap enough that you want to cover yourself with an extra board, go for it. You may not need it, but you can probably get rid of it later if you don't.
"I'm not sure what is a fair price since few are available for sale though I am guessing many are just languishing in people's collection ...
Put another way, what price would you consider a "steal"??"
Look at Sold items on eBay and look for the earliest ones since they only go back for a short time. I guess I would consider anything under $200 for an EW Standard or $250 for an EW Plus to be a pretty decent price, and probably hard to find on Ebay, but maybe not on Craigslist or a word of mouth deal. Personally I would stay away from the older units (Big Briar), but if it's a real steal you can probably pass it along if it doesn't work out. Unfortunately I think a lot of people look at what others are currently asking for prices on eBay instead of what they have previously sold for, and that creates an inflation that you see now.