"What will you buy?"
kau = buy (dictionary form)
kaimasu = I buy/ I will buy kaimasen = I don't buy / I will not buy kaimashita = I bought kaimasendeshita = I didn't buy katteimasu = I am buying katteimasen = I am not buying kainasai = buy it! kattekudasai = buy it please and more...
The difference between 買います and 買う is, aside from the fact that you find 買う in the dictionary as the name dictionary form implies, the level of politeness. 買います is the formal form of 買う when used in a sentence. 買う is a more casual way of saying it.
Apparently, buy is a transitive verb since を is used to mark objects for this purpose.
So, to say 何は...would be sort of like "as for what, you buy?" Doesn't make sense.
I do not understand where the "will" part occurs in the sentence. Shouldn't this mean "What do you buy?"?
A comment in another forum stated that the Japanese have only a present/future form, instead of separate present and future forms.
何をかうの? There are more ways to say it but this the most common kind of informal I guess.