Don't buy in to the "IBM Snobbery" so many do at first. So long as it is in good repair, and not a Terminal keyboard, a Model M of any stripe will get you started. Most you find will be functional. I originally gathered from reading a lot that the Lexmark and Unicomp boards were somehow "inferior" to the older IBM made units. After buying and using ten of them, I have found that the Unicomps and Lexmarks are just as satisfying - if a little different - to type on. Feel does vary a bit - key pressure required, etc. But until you have "one of each" you won't really know what is best for you. Clickykeyboards (and others) have spare keys, and the "blue cube" and ZioTek adapters, to adapt them to USB.
The good news is you can try as many as you like, then resell the ones that are not in the top two for you, and seldom lose much - you may make money! Unless you pay too much, you will always be able to get your money back on a Model M. Enjoy the quest - for the one true keyboard. "The Model M".