Any Japanese 600, Triumph or MV 675 triple, or Ducati 848 are middleweight bikes. These are the most popular classes with the most people. The R6 being the most common ride out there. For parts and setup availability that is the way to go.
There is a good argument for getting started in the LW classes, get a Suzuki SV 650 or Ducati Monster 796 or 1100, throw on some tires and go race. LW classes are more "gentlemanly", usually smaller grids, easier on tires, clutches, and other consumables. Also you can learn how to ride on a lower HP bike, learn the art of corner speed and throttle application without killing yourself.
Don't even think about racing an unlimited bike as an amateur. It should not even be legal. The Kawi ninja 250 is a good cheap option, but you will get bored quickly, those bikes are slow. Unless you happen to be in a region where there's 20 of them racing :)