We are like you and didn't want to buy 150 points up front, so we started with a smaller contract, but I think it was more like 100 points. We didn't have a real hard time finding/buying it, but as mentioned, they do get snapped up pretty quickly, so you have to monitor the major reseller's web sites daily and be ready to move quickly!
The disadvantage to me has not been a big deal until this year when Disney has started enforcing their rule of only being able to transfer in or out points from your contracts 1x per use year. We ended up buying 2 smaller contracts after our initial contract, and if we have a 2BR booked with other family, we need to transfer points since we don't have enough points in one contract alone to do so. Now there are different ways to handle that kind of transaction, and they don't necessarily mean transferring points, but it is helpful if you need to do so. It is better to buy a small contract from a reseller to get started, then when you want to add on, to buy from Disney so you can avoid being penalized like this. (It actually pains me to say that since we have been big fans of resale).
Good luck!