These are my thoughts on buying a Theremin as a newcomer.
Buy a relatively cheap one, around £100 if you are not certain whether the Theremin is right for you.
I would recommend the LV4 as it is far easier to tune than the Open Theremin. The Theremini is not recommended.
If you are certain you and the Theremin are made for each other, then price wise you can go up to about £8,000.
You can get a very good Theremin in the £750 to £1500 range.
It would seem that Moog have stopped producing Etherwave Theremins, so you will only get second hand now.
My recommended choices are - D-Lev Theremin (long waiting list) or a Theremin from Dominik Bednarz. Mine is a Subscope Voicematic V120 with all the extras. Price is determined by which modules you want, but range from about £700 to about £1400.Contact him at bednarz@subscope.de
These are just guidlines, ultimately the choice is yours. I researched and bought according to which produced the best sound in my opinion. You should do the same!
Apparently there are tuning problems with the pitch antenna.