Actually, my home studio is used for very similar work so I can probably help you...
Let me start by saying you can make a record...real productions...with the setup below. This is overkill for a "demo" studio...if you want to make demos, get an MBox and 2 SM57's.
Since you're only going to need a few channels, I think a good start would be:
Apogee Duet ($500)
So, you're at $2200
You'll need mics to plug into that setup...the Shures are always a good way to start, and they are mics you'll never grow out of..
The 57 around the 12th fret with the SM7 on vocals should be great for the singer/songwriter stuff. All 3 mics should work great for rap vocals.
You can get all 3 for $550 new. Now you're at $2750.
You'll want to make your room sound decent...A few GIK screen panels
(self-standing moveable "vocal booths") and a few absorbers ($800) should help. No construction necessary and these make a HUGE difference. I know.
By the way, this is not for "sound proofing." You'll still blow up the neighborhood with loud amps, drums, etc.
You'll need monitors and a few pairs of phones, and (maybe) a "hub" to interface your monitoring setup together...this is a good idea, especially with a Duet that has only one knob to control everything and one headphone out. The Presonus Monitor Station ($350) does this well, and it has a talkback mic which makes producing a lot nicer.
This, like everything else audio, is not just based on quality, but application and taste...and really what you get used to. I have been using Event 20/20's (which get unfairly s*** on in this forum) for about 8 years...they were $900 new. They're discontinued and can be had for $300 for the set on Craiglist...that's a steal.
That said, you can look into something new like the Adam A7's...that'll be $1200 for the pair.
You'll probably want a few pairs of phones...I use the Audio Technica ATH-M50's which are fantastic, both quality and value...It'll be $320 for both.
So now (if you go with the new Adams) you're at $5,420. Not bad.
Put aside a few hundred for cabling and miscellaneous (a book to teach you how to use Logic, a few cheap stands, lava lamps, etc.), and you're at $6,000....
That leaves you another $1,000 for a "money" preamp....I'd go the Great River ME-1NV. If, however, you are a singer songwriter without a decent axe, take the 1K and pick up a used Martin D18...that'll do a lot more for your studio than a mic preamp.
And that setup is t*ts...good luck.