You should contact Dominic Bednarz either by personal message here or through his website if you are interested in learning more about what he offers. I am not aware if he publishes any prices, but he seems to have a pretty defined price structure for the different models and options if you ask.
Dominik built a custom Subscope for me after I had been playing an Etherwave Plus for about a year, and it was my favorite theremin until recently. It had some really nice features and a somewhat different range of voices from those of the EW.
Being custom built though, it will be more expensive than the EW, and since it has more knobs it could be considered a touch more intimidating to start on. Since the EW is so widely available new and used, it would probably be the least expensive instrument that would be suitable for a musician. That said, I would highly recommend the Subscope as an instrument to move into later.
Regarding linearity, the Subscope and the EW have quite similar linearities at the mid- to high-end if you play with a Caroline Eyck style of open-closed hand fingering. Other playing styles may show some differences between the two, but for simplicity I would say that neither is better than the other in all cases. Both compress slightly at the high end compared to the mid range. The stock Subscope has a much more linear bass range down to zero-beat, where the EW tends to compress a bit and lock up near zero beat. But many people have Thierry Frenkel's ESPE01 mod or something similar installed that greatly extends the range into the lower octaves while also improving some of the harshness of the sound. A well-tuned EW can have very good linearity and a pretty good size pitch field that doesn't feel cramped at higher pitches.
Another thing that I really like about the Subscope is a telescoping brass tube that slides over the pitch antenna for making large (sometimes seasonal) adjustments to keep the pitch knob range centered. This allows coarse, less frequent adjustments to be made with the slider while allowing the pitch knob on the front panel to perform as a finer adjustment. By contrast, the pitch knob on the EW covers a wider tuning range to be able to reach proper tuning with the natural variations that occur over time or with changes in humidity, but this wider range means that the control can be overly sensitive to adjust day to day. And even with the wider range the EW can sometimes go out of tune beyond the range of the knob and require removal of the cover and adjustment of the internal inductors to recenter the tuning of the front panel knob. It is possible to make an antenna slider for the Etherwave as well that will in may cases prevent having to go inside the cabinet to retune, although without modification the pitch knob will still remain touchy. And there are actually more variables to consider for keeping any analog theremin optimized other than simply keep the pitch knob tuning within range.
Short answer: An Etherwave Standard or Plus is an inexpensive and quality product that is good for starting on as well as for playing professionally. There will always be other theremins out there to expand into later. Others will have to offer opinions on the Theremini, but my impression is that you may be better off going directly to an Etherwave.