You can barrow all of your points from the next year.
You can also bank points not used into the next year, but there are banking deadlines on the amount of points you can bank.
The flexibility of
DVC is what sold us. Not only the ability to stay at almost any Moderate or Deluxe resort at WDW, but Vero Beach and Hilton Head (both DVC's). You points go further at DVC hotels, but the flexibility is there to stay at the Poly, GF, Comp, etc.
You can also trade out your points to stay at a wide varity of locations around the world. We stayed at the Grand Hotel for two nights this summer and used points.
I would start by deciding when you normally go, decide on "standard" room size, decide on length of visit and calculate the points required. This would give you an idea how many points to buy. Remember, you are not locked to anything!
I also recommend (as will most on this board), buy where you intend to stay most often. The 11 month reservation window will give you the best chance of getting what you want.
I would not recommend purchasing DVC if you don't like to plan your trips in advance (7 - 8 months).
Hope this helps...