I’m all for QRP operation. The rules says something about using no more power than is necessary, and that’s a good idea for a number of reasons. It’s probably a good idea to limit your RF exposure as much as possible and the lower power that you run, the less chance you’re neighbors are going to hear strange sounds emanating from their stereos and computer speakers. And, of course, when operating portable, running low power will help preserve your batteries.
If a station is not mobile, once a QSO is established, the QRP station should QRO if conditions are marginal. It certainly would make it easier for the QRO station operator. I have been in many QSOs where the QRP station could hear me just fine, but it was a bit of a struggle for me. The courteous thing to do would be to switch on an amp at that point.
Check out the HF Amplifiers page on the Communications Concepts website. They sell parts for amplifiers based on Motorola RF transistors, that are described in the Motorola app notes. There’s an amp based on the AN762 kit that outputs 140 W or 180 W and an amp based on the EB10 kit that puts out 600W.