The 15" PowerBook G4 retails for $1999, so $1700 sounds like a good deal. You could always encourage your friend to go a little lower, but that's just my opinion. As for future software being obselete, you don't have to worry about this for a few years. All future software will be Univery Binary, meaning it will run natively on either a PowerPC or Intel Mac. Eventually future Mac applications will be built to run only on the Intel Macs, but like I said, this is at least a few years away, so no need to worry about it.
Does that help?