They are excellent cars, well worth every penny paid for them, and then some. The 23K being asked is reasonable, but talk him as low as you can.
I wouldn't get a new GTO older then a 2005, as in '04 the had "only" 350 HP, plus a few other, smaller, changes that take away a bit from the overall package.
Because they are Aussie built cars, the GTO is more rugged then a comparable American, Asian, or Euro car.
For 2005, there were few options, you had a choice of exterior colors, and on some, interior color, auto/manual, hood scoop delete, 18" chrome wheels, and an "Action Pack", which included a different front air dam, side skirts, and rear fascia, as well as new exhuast tips and grille. They have <i>plenty</i> of oomph, and one of the best stock exhaust notes around. They can be a bit unsorted or broken roads whilst turning, but generally, the independent rear suspension works well. One thing you'll want to do, if you get it of course, is get an CAGS (somthing like that) eliminator kit..like the old Firebirds and Camaros, at anything less then near WOT acceleration, the car won't let you shift from 1st into 2nd, you have to go to 3rd. This is an attempt to improve gas mileage, but really it's just an annoyance more then anything else.
The car is pretty well optioned out, hence the lack of options. High-quality leather in high-bolstered sport seats, 4 bucket seats, 6-disc CD changer with a decent (for stock) sounding stereo, steering wheel audio controls, electric seats, etc.
Main complaints about the car are that on the instrument cluster, there is a bright green light behind the letters "MPH" on the speedo, which never turn off (unless the car is off), and do not dim when the headlights are on. It can be distracting. The rear spoiler also pretty much makes the inside mirror useless. You see little out the back. Not that the spoiler is big or anything, but the short vertical height of the rear window, matched with a average size car mirror means poor rearward visability. Being that it is a foergin car, there are a few "characteristics" that are normal overseas, but not to us Americans; there is no central lock button, the locking of the doors is controlled through the manual use of the door lock pin on the drivers door (I belive that later, a button was added, but I think that was for '06); if I remember, there are either no cupholders, or very small cupholders. Like I say, normal for cars outside our borders, but unheard of within.
One thing that is very novel on this car, which I wish other makers would catch-on to, when you press the Traction Control button to turn it off....it actually turns off. All the way off. It doesn't go into a "almost off" mode, it's off off. Be careful driving with T/C off, the GTO has plenty of throttle induced oversteer, but luckily, it's fairly responsive to steering input and off-throttle use. A modified GTO is the current American champion drifting car...
Just about the only complaint anyone has with the car is the looks, which stem directly from the use of the name GTO. If it were called anything else, people would more then likely sing the car's praises from the highest mountains; but, since the car isn't retro styled, it gets a bad rap. Don't let that fool you. Exterior looks are easily changed.