That was exactly my situation, I saw a 2008 GP, liked the way it looked and bought it. I knew nothing about supercharged or anything, I was disappointed when I found out what was out there that I couldve had with a little looking. My first GP was NA, and base in every way, no monsoon, no sunroof, no heated seats, nothing. I drove that for around 3 years and I was happy with it, but then I started wanting to go boosted.
Bought my Comp G, 2004... monsoon, sunroof, HUD, heated seats, satellite radio, it's literally got everything and I love it. Monsoon is good, I have an aftermarket head unit with factory speakers which is what I recommend unless you're gunna replace the amp and go with a sub. Aftermarket speakers have less surface area so they put out less bass, they're made to be used with a sub.
Regular mpg's combined city and highway is about 21.5 with lots of mountains, though once I took a trip to NJ which was like a 400 mile round trip, all interstate and I averaged about 28mpg doing 65-70 the whole way.
Definitely use 91+, if you put in less than that the car will run but you'll end up with even worse mpg's and paying more in the end, a total fill up on 93 is only like 3-4 dollars more in the end anyway as it was said above.
Everyone always told me modding NA is a waste of money... I can kind of see what they mean now because you have to spend over 1000 dollars NA just to be equal with a stock GTP. The minute I got my exhaust mods done on the GTP I understood why people said that, there's no comparison to NA. But I can also see where someone would have an NA car, not care enough to go through the trouble of buying a new car, and just want to do the work because they enjoy it...