I haven't come across many people looking for 50 points as their only contract. Most pick that up as an add on. But points are points, and all get used the same way. Maybe your BIL likes having the kitchen and extra space and uses 1 br. Maybe he invites friends and gets 2 br. There are all kinds of ways to use the points.
Many might tell you that Studio's might be tight for a family of 4. We had originally planned to buy and stay in studio's, but we did the math and can afford more points to stay in the bigger room. Personally, I don't see a huge difference between a studio and a Standard hotel room. You get a microwave, but you replace a bed with a sleeper sofa (who cares it's for the kids right!

) OKW they say has 2 double beds instead of a sleeper sofa. The only downside I see with your plan is the most you could ever use at 1 time is 150 points. But again, it is up to you how you travel. We bought 125 and I thought that was going to be too much as we originally looked at 100.
The real questions you need to ask yourself is, where do you want to buy? Price is clearly one of your concerns. SSR is one of the cheaper resorts in terms of buy in AND dues. You also need to plan a desirable UY. What PREMIER times would you be thinking? Christmas and thanksgiving with magic season you would probably want a June UY. If Easter is more your thinking, then you would want Dec to April UY.