I think 150 points is a good start. You may find that once you take a few DVC trips, that 150 points may not be enough. It really depends on how you vacation, and what kind of accomodations you want to stay in. I will say, if you stay in a 1BR unit, you'll never want to go back to a studio!!!!! When we had just the 200 points, we would stay in a regular resort for the first 3 days which were usually over a weekend(Friday and Saturday nights are the most expensive DVC point-wise), and then we'd move to the DVC property on Sunday or Monday, and use our points for the less expensive weekday nights. I enjoyed that because it was a nice way to get a little "taste" of a regular resort without spending a fortune. It's a lot easier to justify the deluxe hotel stay if it's only 2 or 3 nights as opposed to 7 or 8 nights. IMHO, Disney made it very easy to move..they transferred your belongings for you, ad I never found the check-in processes very odious. If you're a nurse, teacher, or government employee, the Swan and Dolphin have a great rate of $129/night (although then Disney doesn't transfer your luggage I don't think). We did always start our trip at the regular hotel and move to the DVC property though, because we usually booked a 1BR DVC unit and so I was able to plan packing to take advantage of the washer/dryer that's in the 1BR and bigger units.
DH and I bought our first contract in 1997 with 200 points at OKW. At the time, only OKW and BWV were available, and OKW was cheaper, both in cost per point and dues. We looked at both properties, and our basic thought was "If we could never stay anywhere else on Disney property again except for OKW, we would be very happy here". Of course, that has not been the case, since we have stayed at BWV and BCV, and had VWL booked for early May this year but had to postpone due to my father having eye surgery. Truthfully, we haven't had much difficulty getting a reservation at whatever property we wanted. We do go at "off" times of the year(October/November or January/February) usually, but I have spoken to Member Services recently about re-booking the postponed May trip for July, and they have said there is a lot of availability, especially at OKW, which works fine for us.
Late last year, we decided that we wanted to have another contract, so we bought a 230 point OKW resale through The Timeshare Store. Pat Spell was our agent, and she did a great job, everything went very smoothly and quickly, about 6-8 weeks from start to finish. My DH is a realtor, so we had more of a comfort level maybe since he was in the business and could head off any potential problems, but there really were no problems. As I said, very smooth. I wouldn't hesitate to recommend buying a resale, and you can sometimes get them for a better price than buying direct from DVC, but if you're comfort level is better if you buy from DVC, or the DVC financing offers work for you, then I'd buy from the DVC. I think it's a matter of personal preference.
I'd try it with the 150 points for a while, get the "feel" of working with the points and the times of year and related point values, learn the ropes a little, and then decide if you need an add on or another contract. That's how we did it, and it worked for us.
As an aside, if you think you will be spending more than 8 days in a 365 day period at WDW, then it pays to buy an annual pass.