That is how we started out as well. We bought VWL to start (50 points) and then added on at BLT.
We ended up selling VWL because we realized we just wanted to own BLT, but it was a great way to get in and start with a small contract.
Plus, once we were members, the points we added on at BLT were a bit cheaper because we qualified for the discounts with smaller contracts.
During the late Spring/early Summer, they were allowing new buyers in with only 50 points through Disney. I don't know if this is still an option but if you know you want BLT, I would at least check to see if you can get it through Disney.
The price might be a bit more, but many of the smaller BLT contracts available via resale seem to be stripped of points until 2012. If you buy direct with Disney, you will start out with 2010 UY points.
Plus, you could end get the UY that fits your travel as ideally, having a UY that falls right before you may typically travel offers you the most flexibility in terms of what to do with points from a cancellation or a change.
In your case, for fall trips, I would probably look for a Sept UY. And, if you buy through Disney and have a Disney Visa, you can put the contract on that and get 6 months, 0% interest.
We just did our newest 30 point add on this way. Instead of having to send the money right now, we get to hold on to it for half the year.
Good luck!