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
Remember は is the topic marker and while the topic is often omitted it still exists in concept. So in this case the question is for "What will YOU buy?" So there is an implied あなたは at the beginning of the sentence, basically the particle は is already in use.
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.
There is no future in Japanese language, usually to differ from present words like 'tomorrow' or 'later' are added. If there is neither context nor words implying the tense in the sentence both, present and future, could be viable answers.
何 flexibility really makes me feel like its not something that should ever be used in a real conversation. Such as saying 'Aein't' or 'cool' or splitting infinities, or how the word 'like' (though other forms such as 'likened' are okay) is forbidden in anything formal.