What’s the difference between the present progressive tense are buying and the simple future tense will buy as used here?
- Next week we are buying a new car.
- Next week we will buy a new car.
"We are buying a new car next week" means that you decided to buy it some time ago and you have arranged everything for that (made an appointment at the car lot, took a day off to go there, arranged with your wife of brother to accompany you, etc.). The Present Continuous in this sentence means not just plans but arrangements. 95% that it will happen.
If you say "We will buy a new car next week" that means that you only have such an intention. You did not do anything yet, you're just thinking it over. 50% that it will happen.
I think the difference is mainly in tense. "are buying" is present tense, as in you are definitely buying the car, it's been decided and you're, in fact, at the car lot writing out the check for payment. "will buy" implies a future purchase, one that you've decided on, but you're not at the car lot with your checkbook yet.