Once you close and are in Disney's system, you can start booking vacations assuming you have available points.
Keep in mind that December is a very popular time for DVC stays (the entire month) so much of the DVC resorts have already been fully booked at the 11 month window in January. That doesn't mean that you won't be able to get a room, but I wanted to make sure you knew there is a good chance you might not get something without waitlisting.
Borrowing is done at the time a reservation is made, so if you can make reservations then yes, you can also borrow the next year's points.
Much of this has to do with what points are available for you, when your UY is, etc.