Depending on the exact model you choose, an SN95 will probably have a cheaper purchase price than an LS1 F-body, but will be more expensive to modify to the same level of performance, IMO.
As for weight: when I began looking for an LS1 car, I weighed a stock, 6-speed, T-top Z28 and it was 3333 with the T-tops removed. (Anyone else bring wheel scales when you go car shopping?) I don't remember how much gas was in it. My almost fully prepped ESP Z28 is a '99, hardtop 6-speed and weighs 3365 with 1/8 tank, but that's with 17x11s, 315s, SFCs, big sway bars, and other things that bump up the weight a bit. I can probably shave 40-50 pounds off the car with a smaller battery and lighter clutch/flywheel setup.
Mileage is in the high 20s. In 6th gear, the car only turns 1500 rpm at 70 mph.