To get a complianced ST165 for less than 8K is a pretty good find - although condition and km's are a factor. Mine had done 120000 and was in great nick and I picked it up fully complied for $8,500. Note there are several bits on the GT4 which are totally unique to the car and hence you may find yourself waiting for bits to come from japan when certain things break! But that's just a typical import I guess
Honestly, the ST162 is a very nice car especially for the price. But I would (and have) put my money down on the GT4, and having driven both - I can say that if you can find a good nick GT4 you will be in for one very smooth, reasonably quick ride. I'm yet to take both the cars out on the track so I really can't tell you much more.