Will you be roasting the pumpkin? A 5-6lb pumpkin will yield 2-3lbs of meat after cooking for 30-60 min. I'm fairly certain most people include the pumpkin in the mash. I opted for adding it to the boil for 90 min. Something I learned, that I didn't anticipate - the pumpkin soaked up some of the sugars form the wort. Since I removed the pumpkin at the end of the boil, I wasn't able to 'sparge' or rinse it to help move the sugars from the pumpkin back into the wort - although I gave it as good a squeeze as I could (it was freakin hot though).
Most pumpkin recipes I have seen use some or all of the following: allspice, nutmeg, clove, ginger, cinnamon, and vanilla. With the exception of vanilla, most call for 0.5tsp to 1.5 tsp. (I went with 0.5 tsp, since I can always add more spice @ bottling time - but you can't un-spice it). Vanilla, I've found 2 whole beans for 5 gallons seems to be the norm.