"Which sofa to buy?"
Translation:Quel canapé acheter ?
It looks like qui is when it refers to the subject of the subordinate clause (IT cost me 150), whereas que is when it refers to the object of the subordinate clause (I is the subject in the "that I prefer" subordinate clause, the que refers to the skirt, which is the object of the verb prefer.). Qui is a subject, que is an object.
No you can't. You would write "pour acheter,,," if "acheter" is your reason for doing something. Example:
Pourquoi vas-tu au magasin? Pour acheter des pantalons. Why do you go to the store? To buy pants.
Note that exactly like in English you wouldn't use it without a complement. "I go to the store, to buy." doesn't sound right.
Generally, you don't put "pour + inf" where you could not say "for + gerund" in English. So you couldn't say "which couch for buying?" but you could say someone is going to the store "For buying pants." Even though it is a bit awkward, it isn't wrong. Anyway, that's how I think of it. Sometimes it is useful to think of the French infinitive as an English gerund. Hope it is helpful.