In order to get the present tense, you need to find the imperfective counterpart. In case of "koupit", it's "kupovat". Thus, the present tense "what are you buying?" as well as "what do you buy?" is "co kupujete?"
You're right that "mně" could be stressed more. The text-to-speech notoriously has problems with question melody. But on the other hand, this particular one doesn't sound too unnatural. Word stress in speech generally isn't very strong in Czech.