1) Bigger IS always better
If you get a 75 and want to invest more into the hobby, you have to get another tank, but if you get a 90 and want to invest less into the hobby, you just dial down the light, the bio-load, and the like.
2) I've never dealt with GLA, but i've heard good things about them.
3) Lets see if i can get all the stuff you'll need for a CO2 system right. Tank, regulator, bubble counter, needle valve, diffuser/reactor, tubing, and a drop checker. I've never dealt with pressurized, so i'm just going off what i remember from this forum.
4) There are ways to set up a sump so that CO2 outgassing is minimized, but if you want to really reduce outgassing, look at canister filters.
5) remember that when looking at the lighting, T5's are about double the light output as T8's or CF's for the same wattage.
Other things) What sort of substrate are you gonna use? What style of aquascape do you want to do? What sort of flora/fauna are you thinking about?