"Τι πρόκειται να αγοράσεις;"
Translation:What are you going to buy?
Τι θα αγοράσεις is more common?
Yes, it is. Πρόκειται implies a plan being in place, a firm decision and I would argue it's a tiny bit formal in the affirmative, as in an announcement. When speaking casually, you just use 'θα'. (Not to mean that 'θα' is in any way informal itself; it's just the emphasis attached to πρόκειται that makes the difference.) You could use πρόκειται to nag to someone who has promised you something but has not delivered: "Πρόκειται να το φτιάξεις σύντομα το πλυντήριο;" - it was a deal, a plan but the promise/intent does not seem to materialise.
If you go for "δεν πρόκειται να" though, it ranges from quasi-formal to "No way!". :)