If you buy through Disney, you can actually set up a reservation right after making a down payment meaning you could make the reservation very soon if your purchase today and still be within the 11 month home resort advantage reservation window. Also, if you tell your guide what you want as a reservation when purchasing and, and it is at the resort you are purchasing,and you tell that guide that you are unlikely to go through with the purchase unless you can be assured of getting what you want, that slaes person will get it for you.
If you buy resale (far less expensive per point), you cannot make a reservation until about a week after you close when Disney gets you into the system as a new member. As noted above, that could easily put you well into Sep or even Oct, if you were to puchase today, before you could call to make a reservation. Easter time is high
DVC demand time and by the time you call, there will likely be many things already gone. Most likely you will be able to get something at SSR other than treehouses because rooms there almost never fill before two months out if at all. Likely there will also be availability at OKW and for savanna and standard view at AKV. Anything else faces a significant risk of being gone.