Hi, John. HP offers no Mac drivers for the IIp or IIp+, probably because they were built with only a parallel-port interface, which Macs can't use. They are not PostScript printers, so even if you could connect yours to your Mac, it couldn't use the Apple or Adobe PostScript driver that some HP printers (e.g. my LaserJet 4M) can use.
I believe the only way you'll be able to use the IIp+ with any Mac will be to locate a used Infowave
PowerPrint cable-and-driver package (the cable is essential; the driver alone would be of no value). But that will only help you on a Mac that can boot into OS 8.x or 9.x
and has an ADB port, which no iMac does. And when it was new, PowerPrint was expensive: it cost more than many low-end printers, so most people just replaced their printers instead of bothering with PowerPrint. Even PowerPrint won't serve your needs unless you have an old ADB-equipped Mac to run it on, so you should probably plan to use a different printer instead of the IIp+.