If you buy through Disney you will get the same Use Year. Call your guide and ask to be put on a waiting list - the time varies according to what they have available, but you should get your points within a reasonable time frame (I'm thinking 6 months or so.) If you buy a resale, you have to take the Use Year associated with it, so it's a matter of timing and luck. Sometimes you can find a 50 or 100 point March Use Year Beach Club contract right away, and sometimes it might not come up for a year or more. (I'm being generous with that example, as I haven't really seen a contract like that in a long time, but I suppose it's possible.)
I would go through Disney - it's quick and easy, and there won't be that much difference in the cost. If it's an add-on, you won't have to pay closing costs, either.
Good luck - I own at BCV with a March Use Year, too!!!