I bought the car as-is, and it had (and still has) some serious problems with it. The CV joints were about torn up, and had found out the person which had previously owned the car had driven 10,000 miles over the past seven years with the bad joint. It was not a very expensive fix, but required a good deal of tools and an entire weekend of spare time.
The transmission is missing fourth gear; I am assuming it is the shift fork, which means I must get that rebuilt. That is not at all cheap.
The turbocharger was going out when I bought it, and had finally started making noise recently. Surprisingly, the turbo isn't hard to get to, and there are places which will sell you a new stock turbo for as little as $275, when you turn your old one in. I recommend spending a few hundred extra and grabbing a performance turbocharger rebuild, since the stock turbo isn't that robust and will wear out relatively quickly.
This car's main plus is its performance. Once the turbocharger spools, you are off like a rocket, and the car handles flawlessly. With its all-time, AWD system, the Celica is surprisingly easy to handle in low-traction situations (gravel, snow, etc) once you learn its driving mechanics.
The interior is thoughtfully laid-out, and while not as ergonomically pleasing as most newer cars, is definitely not a slouch in the comfort compartment. However, this is a drivers' car; the driver's seat is extremely comfortable with multiple adjustment and adjustable electric lumbar support. The passenger seat doesn't have that, and feels a lot less cushy. Rear seating is a joke; it's more than a bit cramped and uncomfortable back there.
The Celica GT-Four (or Turbo All-Trac) is an extremely rare car, and as such, some parts are hard to find. Since it's just a rare trim level, a lot of common parts are used, and in this aspect... it's great. It's also very cramped under the engine bay. Coupled with the fact that the engine runs very hot, with insufficient airflow through the bay, on older GT-Fours you will find hoses badly in need of replacing.
All in all, the Celica GT-Four is an extremely rare car which isn't all that expensive right off, but if not looked after, can become a maintenance nightmare. But, it's an extremely fun car, and being one of a scant few on the road with one appeals to me... better than driving a hopped-up car which everyone has.