if you buy property off-plan, you buy a house, flat etc that is just starting to be built, with an arrangement to pay part of the cost of the property at that time and the balance when the property is finished
buy something ↔ in
to buy something in large quantities:
Companies are buying in supplies of paper, in case the price goes up.
buy into something
1informalto accept that an idea is right and allow it to influence you:
I never bought into this idea that you have to be thin to be attractive.
2to buy part of a business or organization, especially because you want to control it:
Investors were invited to buy into state-owned enterprises.
buy somebody ↔ off
to pay someone money to stop them causing trouble or threatening you [= bribe]