we bought into
DVC (BWVs) with 270 points, when our kidlets were little. we would stay in a 1 bdrm for 8-12 days, then a 2-bdrm every other year for a week. we added on 60 points at BCV, then 100 at SSR, leaving us with 400 points.
now DH and i go in oct, and 27yo DD and i in may, each for about 10-12 days. we found we prefer studios (more than enough room, without feeling overwhelmed - we don't cook, and enjoy the pool while doing laundry so no need or desire for a full kitchen and washer/dryer

).
we rarely wind up with extra points, but found it easy to trade or rent when it did happen (normally, i am contacted by previous renters/tranferees asking if i've any points available - so no searching on my part).
i wouldn't recommend buying with intent to rent; as others said, the market might not always be there, and the small profit might not be worth the time and hassle of trying to find renters (initially, you will most likely find a lot of looky-loos who are just window shopping with no firm plans but are simply "dreaming of going to WDW one day", who ask multiple owners to check the same dates, who try to get points for a rediculously low price, with creative sob stories, who have no idea how DVC works, etc).
david's is an option, but i've noticed he has gotten a lot more selective on the resorts and quantity he will rent.
hth
