Decks, Patios, Porches and Docks - Questions on expanding existing patio

I have an existing patio. 25 years old. Great shape. Aggregate concrete 4" slab in the shape of a guitar body split vertically (two humps). On the far edge the ground slopes up gradually until it is 2.5' above the level of the patio. I am looking to extend the patio about 5' around and then put in a block retaining wall around the outer edge of the patio. I've already excavated down to the level of the bottom of the existing slab. Questions:

1) If I want to have aggregate concrete poured, do I need to excavate further and put in gravel? Or can they pour the concrete on the existing ground (hard, rocky soil)? I'm in a wet part of the country - would gravel under the new slab be necessary for drainage?

2) Would it be better to have the patio poured first and then put in the retaining wall - or vice versa?

3) What ballpark figure am I looking at to have 200 sq. ft of aggregate poured 4" deep, presuming I've done the excavating? Note that it's in my back yard - about 60' from my front curb. (One neighbor said I'm looking at $600 and another said $3,000 - big difference!)

Thanks.

Call a couple or three concrete contractors and get their estimates. Personally, I'm guessing that BOTH of your neighbors are way off with their estimates. It's not going to happen for for less than $1K, but it shouldn't be more than $2K.