I currently use a booster seat for a 4 year old in my 02 GT and used a car seat prior to the booster seat. It fits ok, just a little harder on your back bending to put the little one in. Now at 4 she climbs in herself. I've always kept her on the passenger side so the front seat could be more forward then mine while driving. The car seats need more room then the booster seats as their legs/feet kind of stick out forward rather then down.
One nice thing about a "small" two door, is the child is within better reach of the driver (when on the passenger side) then in a larger car. Makes it better for handing them stuff.
Most of the rear seat dimensions for the 05 are a little bigger then the 99-04. The 05 will also have the "latch" system, my 02 doesn't.
We have another child on the way, I think I need the slightly larger 05.
As with you, our other car is the primary family car. But i still need to carry the daughter (soon to be 2 children) for dropping off at school/day care.
Before I bought my 02 I rented one to see how the car seat would work out.
go for it!
Edit : rjstaaf makes a good point. My daughter was already in a forward facing seat when I got my 02. I think a rear facing will fit but I'll have to check it out, before I order my 05.