To record a decent sounding demo you need.
Computer, recording softie, interface, preamp, microphones (or record direct using amp sims)
You may also need amps, instruments and midi controllers to record keyboard parts. Drum kit would be wise as well if you plan to add drums to your tunes.
Should have a bit of room treatment to cut down on room reflections. Would be nice to have some decent monitors and plugins to mix with. UAD card looks like a good choice.
Then you could go big time and start buying real hardware compressors however you will need more cash for that.