Keep in mind that the premium package is ~$189 per person, per night as of 2013 and this is very likely to rise ~2-5% per year.
While food is costly, I
strongly advise you on doing the math beforehand. The tours are a nice added touch, but they can easily eat up much of the day, and with the premium package, you really have to do many of them to get the paid value out of the plan. Alternatively, look into splitting your stay, with a 3 nights (the premium plan minimum) on the plan, and the rest of nights differently. This could allow you to have some of that experience, but not be tied down to constant tours and recreation things, while the parks sit there taunting you!
From a dining perspective, we won't know how valuable the dining plans (Quick Service,
Disney Dining Plan, and Deluxe Dining Plan... these are actual dining plans rather than recreation/dining packages like premium and platinum are) will be in 5 years. At the current trend, it will likely be cheaper to go out of pocket. I know that you had sticker-shock at DLR with the OOP prices, but it would still make more sense if you're going to spend less OOP than on a dining plan.
So, as such, start getting a general idea of where and how you'd like to dine for now. When the time comes nearer, you can do the math to see if a dining plan is a right fit for your intentions or not. If it's not, there's a very good chance that it will be cheaper to go OOP, even with the high individual prices at WDW.
Length of stay can be a bit personal and subjective. Some people can feel they did enough within 3-4 days, others can't fit everything they want into 14 or even 21! I'd take the time before your trip to start determining what you want to do and stuff and seeing how long that takes up, rather than determining a time frame and trying to fit or fill as needed.
I know, I'm not all that helpful

, but I
strongly believe in crafting the right trip for you rather than you trying to fit into what I (or others) do! Of course, you have time, and some great resources at hand to help you decide.