If you just want to mess around and get your feet wet you cant go wrong with the Craftsman for a hundred and fifty bucks (There abouts).
Beyond that the two most important things are going to be standard size which is 2x72 in American and 5cm x 2m outside of America. That way you have the best access to belts, everything comes in 2x72. There are some technical arguments for longer belts but once you go beyond this standard size you hit a wall with availability off the shelf. That and for general knife making 2x72 is plenty.
The second is speed control. This is the the single biggest issue when you go beyond hogging steel. One can get by with step down belt drives and all that to slow down but a variable speed is the way to go.
My biggest issue with the KMG is the fact its a belt drive. I like direct drive machines and would not buy a belt drive having messed around with them in the past.
The best deal on a off the shelf ready to go VFD machine I know of right now is at Uncle Als ( riversidemachine.net )
http://www.riversidemachine.net/item14549.ctlg http://web.ivenue.com/riversidemachine/images/grinder.JPG The best grinder out there right now is the Dozier which runs 2,995 plus shipping but it is a knife makers dream of a grinder with a very special belt tension set up. From what I understand AG Russel uses these grinders in his shop as do several knife companies.
http://www.dozierknives.com/index.php?option=com_content&view=article&id=51&Itemid=54 http://www.dozierknives.com/images/documents/grinder1_small.gif Both of those machines made right here in Arkansas by machinist.
The bottom line is you don't need a belt grinder at all to make a great historical or modern knife. Anyone that does not believe that can reference Don Fogg. I know for a fact he was making knives in his basement a couple years back with no grinder at all, only using Sen, files, paper, and stones.