One other thing to consider: moving forward, the maintenance fees for DVC will cost considerably more than the up-front costs, and the more points purchased, the higher the MF costs ... and MF costs can vary significantly between home resorts.
Second, you didn't say if your were thinking of buying direct from Disney or resale - that will make a big difference in your up-front costs (and give you more options on how to use to points).
Third - where you stay will make a difference. For example, I took a quick look at the
points charts, and the "most expensive" weekly rates for the DVC Resorts seem to fall into three groups for a 2BR -
1 - about 400 points: SSR, HHI, OKW
2 - about 500 points: AKV, BCV, BWV, VB, VWL
3 - about 600 points: Aulani, BLT, VGC
So it looks like you would need at least 250 points to cover most of the resorts, going every 2 years ... with 300 points covering them all ...
3BR GVs run about 1000 points for the "most expensive" week, so 300 points would not be enough for the "most expensive" weeks ...
Considering you'll want to make sure you "get what you want" for your larger family trips, choosing your Home Resort and having that 11-month booking window is also an important consideration.
So - as a few others have recommended - sit down and compare a number of scenarios (resorts, time of year, # bedrooms, etc.) and see what works out best.
This is a great gift of "pixie dust" from your son, and will bring many wonderful times for you and your family ...