Here's some additional perspective:

If you get a weighted 88-key keyboard / piano, it may very well become an interest on its own. This is what happened to me; I started learning piano in order to more efficiently learn music theory for digital music production, and now I am learning piano just to learn (and play) the piano. In other words, getting a full 88-key keyboard may expand your overall musicality and lead to you developing new musical interests.

I would also potentially consider another common instrument: the guitar. A decent entry-level guitar ($300 - 600) will cost less than a weighted 88-key keyboard (start at $600 but will probably spend $750 - 1,000 once all is said and done, don't forget a proper stand and chair). A guitar, especially an acoustic guitar, is highly portable, and still wholly conducive to learning music theory. You can learn to sight read on a guitar (e.g., Berklee's Modern Method for Guitar) as well as learn and apply music theory.