I had an 03 Coupe 5AT and the only problem I ever had with it was the stereo problem. This is where the factory amp/stereo draws too many amps on your battery while the car is shut-off and drains the battery, took it in to Infiniti and they fixed it. No problems. I personally had a slight preference to the 03-04's based on interior, and the brembos. The 05-06's interior is pretty much the same with a few small changes, but for some reason, despite the bugs with the 03-04's I do prefer them. Use Kelly Blue Book
http://www.kbb.com to get a good idea of what any vehicle you are looking at is worth and then go from there. Good luck, welcome to Nico, and we look forward to you joining the G family.