From my limited experience with R32s I feel the R33 is a more refined car. The 2.5 has plenty of low down torque if you just want to cruise around and be lazy with the gears. As it is a bigger, heavier car the ride is a bit more plush. I've heard the trade off is that the handling isn't as sharp. It's still a very good handling car, just not as light in feel as the 32.
Going by ads in the Herald etc. there are lots of private sales for less than $20k. Condition of these cars may vary though. It seems most of the cars have been "clocked" so go on condition of the seats, pedals, trim etc if it has lowish kms.
Personally, I think the 3 Year Commercial Warranty offered by most dealers is a rip off. It has too many ways for them to get out of doing the work and not enough inclusions. It is basically an insurance policy for a limited number of mechanical problems. You also have to send in receipts everytime you get work done to keep it active. If they don't like something they can knock it back etc. The Statuatory 3 month Warranty the Dealer must provide has more teeth but it's not perfect either.
You are better off getting a qualified inspector to do a thorough Pre Purchase Inspection on the car.
I bought a totally stock, unmodified '93 model 2.5 years ago and after a couple of minor fixes (timing, water pump gasket and a speedo problem) it never skips a beat. Whatever you get keep it serviced and run it on 98 octane.