For most English speakers, these variations would not be noticed. "I will be buying", "I will buy", "I am going to buy" would all be acceptable phrases. I think in the example given, the phrase "will be buying" indicates intent, while "will buy" indicates a fact. So, yes, they are expecting or planning or intending to buy that car, but the timing and the exact type of car is not yet determined so there is intent on their part but it is not a fact. There is so much flexibility in English that points like this can be very frustrating for the learner who wants to be accurate, yet the native speaker would likely not even notice the difference. I hope this helps.