1) Check menus on allears.net and you'll be able to answer your own question. What we do every time we want to decide yay or nay on the
DDP is to figure out how much it costs to add it on for 1 day. Then visit the menus and pick out what we'd eat. Compare costs, and voila!
2) We had never gotten MM before, but on our last trip (with DS1 -- his first trip!), we decided to do it. And now I think we'll always get it-- we wound up with soooo many great pictures, and had tons of fun playing around with borders, etc. after getting home. We've never hired a photographer to do photo sessions with our son, but we found it so fun to see what the PP photographers would capture on our son's face. One went out of his way and basically created a mini-session with our son on Main Street, and they are our favorite pictures of the whole trip -- those were worth the price alone.
3) Depends on how much you want to do on your own. I love love love playing travel agent for my family, so I'll never hire anyone else to do it since I enjoy it so much. So see how you feel making the reservations, looking for discounts, making your own fastpass+ and restaurant reservations... It's totally doable with just a little bit of familiarizing yourself with the process (esp with disboards!), but if you don't feel up to it, look for a great agent -- and make sure no one is charging you for the service. Lots of advice on that on disboards.
P.S. Be sure to get buttons to wear that say "first trip" from guest services or from your hotel (if you're staying on site, which I highly recommend!) -- you can wear them for your whole trip, and you will get lots of well wishes (and maybe even a treat or 2 along the way). Also you can get buttons if you're celebrating bday or anything else in particular. Have a magical trip!