Running a 7200 RPM HDD is fine. The 80 and 120 that were being used in an SL iMac here are still running ok(the 120 is the only samsung drive in the house still alive, the other three in various other cooler machines died premature deaths). The iMac is toast, but then so is another one that only ever was stock. That iMac was on 24/7 for 8-9 years and was regularly doing some reasonably heavy work until the mid point of it's life, not sure about the stock one though.

There is no way to preserve an iMac without leaving it off most of the time. It just does not work. You are better off replacing the bulging caps on the PAV and replacing the CRT when it gets burnt than trying to 'save' it with a 5400RPM drive.

Optionally undervolt a few small fans and arrange them around the handle inside the case blowing out. The reduced voltage will keep noise low but still move more air than with no fans.