The SS is a great truck, I still miss mine.
Couple of things to help you out:
2003-2004 were AWD only
2005 it was an option but RWD was standard
2006 was RWD only and AWD was completely discontinued
They have the same weak 4l65e in them as all the other half tons so be aware that if you get an AWD version you're gonna be replacing a tranny or rebuilding it. There is no question of IF... just a matter of when you'll blow it up. The stress of the traction all 4 tires putting grip to the road adds is too much for the 65e even if the motor only puts out stock HP.
As for the gas mileage thing... you can expect to see between 12-15 depending on how you drive. This is another thing AWD will play a major roll in. The added drag/weight of the front drivetrain will kill your mpg... my 04 was AWD and I was lucky to see anything better than 10-11mpg... but I was also turning 22's and had quite a bit of work done to the tranny so that might not be the best gauge.
Other small issues to look at with the SSS -
Steering shaft clunk. All the GMs have this so its not a big deal, but it seems to be more pronounced in the AWD's than anything else.
Body cladding waves. 03-04 trucks had a tendency for the body cladding to warp or get ripples in it. Not sure why and it wasn't on all of them, but it was common... so keep an eye out if you're looking at one. There is no TSB for this so the dealer won't help you.
Other than that there isn't much else to complain about. Like I said I loved mine and miss it alot. Fun trucks for sure... good luck finding one. I suggest RED :read: