We did just that. We were interested in buying but could not afford 150 points so we looked for a 100 point contract. We were not paticullarly set on any one resort since we were sure that any Disney resort would be wonderful. We ended up finding one advertised as 100 SSR points. When the agent did the search they found that there were actually 110 points. So that is what we got. It is a good amount for us for now. If we need to and can afford it we can always add on later.
We are planning to try HHI next year and I expect that if an add on is in our future it would probably be there or some other off site location.
I also agree that it may give you leg up on ROFR. As long as the cost is reasonable, Disney has nothing to gain by buying back a resort that they are curently selling. They would have to pay the closing on the purchase and then suck up the closong cost on the sale. It just wouldn't be financially beneficial for them to do it.