Whilst you're trying to decide on the K300 and K350, with a bit of effort on setting up software you could get a virtual K250, the sounds are very similar and the cost is only £4. It can be played via midi or on your computer keyboard. Having owned a K250 I can tell you that this virtual instrument is exactly like the real thing. I've spoken to the creator, nice guy.

http://www.forgottenkeys.co.uk/7_k2x0.php

The K300 and K350 have a few more sounds and rhythms, yes they are better than the K250 of course and the K300 or K350 should be fairly low cost these days. If you like their sounds they are well worth it.