You may want to get contracts smaller than 270, just in case you have to sell at some point, I guess, but I would definitely get enough points at one resort, one use year, to be able to book a "standard" vacation for just your family. In our case, we wanted 7 nights in the value season at BWV. That's 270. We don't want to move in the middle of the vacation (too many kids, too little time!), and I want to be able to book all our nights at the 11 month window. I think the decision to go resale or buy from Disney depends on when you want access to the points and how long you're willing to wait for the right contract. I looked for about 6 months, waiting for a good resale to appear, and did not find the "right one." (That was before we were SURE we wanted more points!) I tried to contact one agency and got NO response, so I didn't try anymore. Taking Shontell's advice about getting on their lists may increase your chances of finding something you want.
We liked the convenience of buying from Disney. The points were available the day after I called, long before we closed. We also only wanted to buy at BWV or BCV, because we want the convenience of being close to Epcot and MGM. Two fewer days to get everyone in and out of the car.
One other comment about multiple contracts - you may find it easier than I did, but it's certainly more complicated to keep track of two contracts and the points borrowed and banked in each than one. I'm not sure how it works to transfer between them, but I understand you can only do that once/year. Ours are not so bad, because we have enough for a full week in one contract. If we needed to use both contracts every vacation, it gets trickier. I think you have to have the exact number of points for one night in one contract or face the transfer issue. You probably want to ask someone more experienced with this than I am.
Good luck with your decision, neighbor! (We're near Dallas, in Flower Mound)