I want to ask about Kawai DP model:
-CP67
-CA91
-CA61
-CA51
-CN42
-CN32
-CN22

What is the differences between each model?
What is the main features of those products?
What is the recommended model to buy?

I tried the Kawai CN 22, 32 and 42 and among these three I would recommend the CN32 as the best "value for the money".
The piano sounds are gorgeous in my opinion.

Well i can tell you apart from a few of them -

CA line = Kawai digital pianos with Wooden key action - something that is thought of as good! since it has the weight and all blah blah.

CN22/32/42 = is the newest CN line so it should have slight better piano samples, but it has a plastic action (which is still not bad, since it is weighted and all)
The CN line is a bit cheaper and is a great digital piano. i would go with the CN32 since it has the cool cabinet look, and i not very expensive.

the CP line if I am right is ensembles, some real big and ugly big screen lots of buttons and lots of weird stuff.. i havent tired them out, but i wouldn't consider buying them if youre just after piano. then i would reccomend CA18 since it is very pianoy.. It has few other sounds than piano. but still has a awesome cabinet thing. and it has wooden keys