They are fairly similar. Unfortunately I don't have the manuals for any of them yet but I have a sales brochure with brief specifications.
The K350, K400 and K450 share the same sound chips but have different panel controls.
I think you would be disappointed with the K300 in comparison with the K350 due to lack of solo sounds and some other fairly important features.
I've updated the K350 page with all of the info I have including the sales brochure. I'll add more info in due course and an instruction manual as soon as I have one:http://technicskeyboards.com/technics-keyboards/technics-k350/
You'll also find this page useful:http://www.synthmania.com/sx-k450.htm
The manual is also available at this site at a cost (no connection to me, I think these manuals should be free but I occasionally have to buy one):http://www.manualscenter.com/manuals/technics/sxk350-owner-manual.html#.VErzYfnF98E