Last I heard, Apple II prices are starting to get up there. But you're right, the missing keycap is a cosmetic problem at the very least and a functional problem at worst. (IIRC, the missing keycap is the reset key. Which is fine if the key still works, since you don't exactly use it while touch typing and you could still reset the machine without power cycling.)

The other thing to be wary of are potential problems. These machines are getting close to 30 years old. Oxidation has been a problem for years. Fortunately that is easy to clean up, but it does take some work. The II+ that I recently gave away had a dying PSU, presumably because the caps were at the end of their life. Again, you can expect to refurbish that component. In otherwords, only spend the money if you are willing to devote time to fixing it.