What Software/hardware To Buy, iMac based home demo recording system Options

[Stevie]

I am singer/songwriter looking for a simple, good quality, inexpensive (in that order) home demo recording system.
 
I would like to use my one year old iMac 17 to record demos to:
 
1)  Put on my website.
 
2)  Send to booking agents.
 
3)  Eventually, use as guides for making commercial quality studio recordings.
 
Requirements:
 
1)  It must record guitar and vocals. 
 
2)  The system must have reasonable quality echo, delay, eq and other effects
 
3)  If possible, I would like to use drum samples from the recording/sequencing program, but if that turns into a problem, I can use my drum machine on two of the tracks. 
 
4)  If possible, I would like to use my guitar as a bass, in other words, record a single note guitar track then cut the frequency in half. 
 
5)  If the system comes with piano, synthesizer, horn or other samples, so much the better.
 
What do you recommend?
 
If I wanted to do commercial quality, what would the price/ease-of-use penalty be?
 
Steve