Actually, if you buy one of the new Intel Macs, you could run Windows either through "Boot Camp" or virtually through Parallels Virtualization software and keep using WMP 11. (but why?)
I think iTunes will do the job for you. Why not download the PC version and check it out? If iTunes PC will handle your RIP'd files, you should have no problem with iTunes on the Mac. If you do have a problem reading the files, there are conversion utilities (freeware) that will do the job.