The FiST will make you change your definition of fun, its that good. I'm coming from a Chevy truck as well (was getting 10-11mpg, I feel your pain!) the gas savings alone will pay for more than you think, in my case its enough for insurance and extra money on the payments.
As a 6'4" 220lb athletic guy I can say in full honesty I'm plenty comfortable in the cockpit (recaro's too, some folks say they are narrow, but I'm pretty broad, I guess my shoulders and upper torso place well due to my long frame...), seat is all the way back and down, but everything lines up like a dream, shift knob right under my palm with elbow on the armrest, drivers door soft armrest pad gets my left elbow which leads my hand perfectly to the steering wheel, no complaints on leg room either, and the addition of a dead pedal really seals the deal when cruising. The potholes here in San Francisco are enough to give James Bonds' ejector seat a good run for its money, and I've yet to hit my head on the ceiling (sunroof as well, you'd have even more room without it).
I think the deal you are looking at sounds solid, even without mention of options. You're basically getting a fully broken in car ready to make your own at a discount that is enough to buy all the mods you'd ever want.
I personally can't yet speak to power with mods, as I'm putting my pennies away to do everything in one shot, but I can tell you in stock form this car is more than enough fun to keep you entertained. There are TONS of threads on here with information on mods and power with dyno's and all that, just dig around some of the subforums to find exactly what mods you are looking for more information on.
I think the aftermarket for the FiST is pretty healthy, I would like to see more in the lighting department (come on projectors!!!) but as far as power, chassis, aesthetics, suspension its pretty well covered. (wheels are lacking a bit, but thats a 4x100 bolt pattern issue, not an aftermarket thing, and there are ways around that as well)
I hope you are able to make a move on the car if thats what you decide to do, let us know what happens!