Without more details (i.e. vacation patterns, hotel preferences, family situation, etc.) it's hard to say whether you should or shouldn't buy into DVC. However, I'm guessing you're a frequent WDW visitor since you knew to post your question here.
With that said, if your family visits WDW at least once a year and likes staying on-site, you should at least consider the possibility of DVC ownership.
Amount of points you need depends on what you are looking to do. If you have a small family, 100-150 pts. might be all you need. If you want to vacation with your extended family or friends or want the possibities of trading out of DVC, you might need 200-250 pts.