Uh, not to be the devil's advocate, but studios have been going out of business left and right.
I doubt that you will get any resale value out of the space, should you sell it later.
I have had homes with darkrooms and recording rooms in them, which did not really add to the selling value of the home later on.
Can you use the potential space for other things, like a band practice room or an exercise room, as well?
A vocal space and a drum space, if you are going to record real drums, will be needed. The online publication soundonsound.com has so much good info. There are also very realistic sounding drums that are computer based now, same with piano. A POD XT pro or better POD gear can actually create great guitar sounds, if you download patches from users instead of using the ones it shipped with. All of these can be used with headphones.
I know because I have great amps and a POD, and record with both. Same for drums.
Having said that, a good neutral recording room with good foam and a nice recording desk and some good headphones and monitors will do wonders for creating tunes for digital computer based recording. ?if you can?t have a great sounding live room, then at least have a good neutral room?. You can create virtual rooms in the computer that can add the ambience needed for great recording (check soundonsound to see how).
I would have to say that it is more about having gear that works well together and the knowledge to use it, that is most important. Getting good tones initially into the setup is vital. A good range of mics and pres that work with them is great, but does not need to be too expensive, starting with good older Shure 58s and 57s.
Not knowing your situation, I hope my 2 cents worth is useful.