If the body and interior are in great condition, i would pay $800 for it.
Absolute worst case scenario, you shell out $350 for a lowish miles 5sfe that will run for another 100k with routine maintenence.
These cars ARE prone to rust on the rear fender wells, if it's started, get it fixed quick, or it'll eat the car fast. If it hasn't started, then cool beans. Another worrysome rust area is where the front sway bar bracket, passenger side rusts out. Resulting in, an AWFUL sound when the swaybar loads up and then snaps against the bottom of the car. It's horrifying.
The motor, if it DOES just need a timing belt, and has been cared for otherwise, should be fine. They're like tanks.
DO take Hess's advice. While i have never personally seen a 5sfe leak here, it does happen.
As for the rest of the car? It handles amazingly well for a FWD car. It has enough power to keep up with traffic, but without making it fast enough to feel like a race car, and promote irresponsible driving. It looks good, she'll like it.
The brakes are GREAT for a stock car. They were essentially engineered for the Celica AllTrac of the same years, so they're overkill in this application.
Parts are plentiful, they're almost sickeningly reliable, so you probably won't need those parts once it's running.
If it was red, it's pink now. 90-91 red celicas had a single stage "Super Red" that faded to pink about 2 weeks after rolling off the assembly line.
What kind of options does it have?
I would probably try to talk down to $500, based on mileage, and that it's not running.
As a reference point, i paid $600 for mine, not running, no rust, perfect body at the time, 162k miles on chassis. It's not a GTS, but the only difference was options, and the GTS is a sexy widebody that's just begging for some 17x9 wheels.
Here's mine: