What do you want to use the theremin for? That should help inform what you need to buy.
I wanted to play tonal music, and got a theremin for my 15th birthday. It was a Gooood decision. It's up to you entirely though. Some people hate the things, others would sell their soul to get hold of an E'Pro. I had some musical proficiency to begin with, and a pretty good ear (which helped), but I cannot tell you in advance whether you'll be able to play the thing. That's the main concern that I had - they're pretty expensive, and there's no guarantee that anyone will be able to play them to their own satisfaction. Some people are pretty good, but get frustrated and give up, whilst others are dreadful, but still adore their playing and find the instrument fun.
I don't think you said anything about your musical background, but if you haven't played an instrument before, and you want to use a theremin to play 'proper' music (as opposed to experimental/atonal woo-woo effects and pieces), then you might want to try something else first. If you just want SFX, then it doesn't matter hugely. Anyone can do that right away, and practice to 'explore' the capabilities of the machine.
What do you plan on using the theremin for? What is (if you mind me asking) your price range? How long've you got?