Hi Cookie,
Welcome to the forum... I'll bet you've been waiting for us admins to say that!
Regarding your floor thickness, I think it depends a lot on the type of construction you use... You could probably use 3/4" and get by without furring strips in the corners... but since you probably need furring strips any way, plan on them, and then go with some nice flat wood for your construction...
1/2" baltic birch is nice and flat. If you don't want to pay the premium for 1/2" baltic birch, you could always consider the 3/4" maple plywood they sell at home depot for $29 a sheet. (I have a lot of that in my Lil Diner!)
And just for grins and giggles... here's what my dropped floor looks like...
it's built out of 3/4" maple plywood, and I didn't use any furring strips, other than the 2 reinforcing bars on the bottom... It's quite heavy though...
Mike...