It sounds like you are trying to decide between buying 200 or 300 points direct from DVC, with the 20 points free for every 100 you buy. If you choose to buy direct, choosing to buy in 100 point increments is smart, as it maximizes your "savings". If you do decide to go with a direct purchase, you could have DVC break the contract into two or three 120 point contracts, which would make it easier to sell if you ever decide you have too many points, or need to get out of DVC entirely. You may even see if you could do them at 50 point contracts, although I wonder if you have to have a 100 point contract to qualify for the 20 bonus points.
As others have alluded to, the perks that come with a direct purchase could be eliminated at any time by DVC, and the only thing that you are guaranteed is the 11 month booking window for your home resort.
We did our initial purchase direct from DVC in 2008 for AKV and financed the purchase through them. At the time, we did not have enough cash on hand for the amount of points that we wanted, so financing was an acceptable route for us and we paid it off in under 3 years.
Since our initial purchase, we have paid cash for two small add-on contracts via resale and we have a one at DVC for ROFR currently. Had I known about the resale market when we first bought in, I may have done that instead, although, as previously stated, it worked out better for us (at that time) to finance. We probably would not have been able to buy for a few more years and we would not have wanted to wait until we could put together enough cash to buy a resale.
Some of the decision depends on how big of a hurry that you are in to "own a piece of Disney". If you already have your next trip planned with the "free" points that they are giving you, then going resale may be frustrating. If you see yourself as an every other year Disney visitor, you have a little time to look at things a little longer. I think the AKV 100+20 offer goes through mid-June, so
take your time.
With regards to using AKV points at the 7 month mark, we have stayed at BLT (10/11, 6/12), OKW (8/11), SSR (1/09 - developer points), and BWV (3/12) all booking within the 7 month timeframe. Now, I know it is highly unlikely that I will ever get BWV or BCV during Epcot F&W or VWL, BWV, BCV, or BLT during the time between Thanksgiving and NYE, and we are OK with that. We really
love AKV-Kidani, and would be very happy if that was the only place that we could stay for then next 45 years that we own DVC. If you really have your heart set on going to F&W every year and staying at BWV, then you really need to own there. If you have to have a Magic Kingdom view at BLT every year in December, then you need to own at BLT.
Really take some time to look at the
point charts to determine how many points your really need. You may find that the 240 is plenty, if all you need is a studio one year and a 1BR the next. Maybe you have 4 kids, and 360 points won't even be enough. Only you know what your needs are currently. Starting small is always a good idea, because it is easy to add more points later via resale or direct.
Read, read, and read some more. There are folks on these boards who are much more educated than me on making this big luxury purchase. Again, take your time and have fun! We have loved every minute of our DVC ownership!