There is a lot of information all over these boards as well as at dvcnews.com that I used to help me make my decision to buy into
DVC last month. You are thinking of all the right questions about deciding about the trip.
First, a stay in the one bedroom for the week at the end of August , at say VWL, is around 270 points. That could change over the years if DVC decides to reallocate the points (which they just did in 2010). Of course, if they add points to one type of room and season, then they will lower something else, but just so you are aware that there can be a points difference. During Spring break, it would be about the same. The studios are slightly smaller (maybe 50 - 75 sq. ft) than the deluxe rooms so if you don't want to worry about that, definitely plan on the 1 bedroom.
So, if you really want to go twice a year, every year, you would need about 550 points.
DVC members do get some perks, but most will tell you not to buy for those reasons as they can add and take them away at anytime. The biggest perk I think for me is the $100.00 discount on the yearly pass. The fact that you go twice a year would make it very beneficial for you.
MF range around $4.00 - $6.00. They do go up every year but I am not positive to the amount. Other members may have a better answer than me.
Contract length is something else to consider, along with picking a home resort where you will be happy. Since you are used to staying at the Poly, you may want to consider the new resort, BLT. It is only available through Disney and has a higher price per point than buying a contract through resale, but right now, the MF are the lowest and the contract goes until 2060. VWL ends in 2042.
You can add the dining plan option to any reservation. It has to be added within 72 hours of check in and you pay when you get there. DVC rooms do not get daily housekeeping but depending on the length of your stay, you get at least trash/towel (if longer than 7 days, you get both trash/towel and a full cleaning once).
Right now, Disney does have some incentives for new members if you have a member referral (if you are interested, just PM me...I can share my info and my guide--he is great!). However, VWL is available through the resale market and the prices are very attractive right now. There are several great brokers. I used TSS when I bought a small resale contract last month at VWL and had a wonderful experience. I then added on more points directly through Disney at BLT. That has also been a great experience.
Lot to consider and lots to read! Good luck and happy planning. I think that if you love Disney and go every year, at least once, the DVC is definitely something to seriously consider for your accommodations!!!