The computer is not really the important part. Save some money and use an iMac. You don't need a nMP for audio. Especially if you are going for low cost.
I think the core of the studio setup for a small studio is the audio interface. How many channels will you be recording at the same time? Are you also recording MIDI instruments? You answers to these questions determine which audio interfaces you can use.
Then after that it's audio preamps and whatever outboard equipment you want. For me, I think only a outboard compressor is needed and the rest gets done in software. Some audio interface come with most of what you need built-in.
Next is the space itself. It has the "work" for whatever you are doing.
I don't think a sub is a good idea. Buy full range speakers. At least buy speakers that go to about 60Hz, not higher then if you need the range for 20Hz to 60Hz, buy the sub. Reason? You want to know where the sound is, you don't want to hear a combined left/right. That is OK for home theater but not for creating the mix.
If price is an issue buy some passive speakers and drive them with a cheap stereo amp (even cheap recovers are pretty good.
The big decision is the audio interface. Presonus, MOTU, and other brands like that are all good..
I'd go for the 27" iMac, you can't have enough screen space.