We took DS for his first trip at 6 months. Although he doesn't remember a darn thing, it was very cool to see him interact with Pooh and Friends at Crystal Palace (those are my favorite pictures of him!!!!), as well as just gaze at everything in Small World and he obviously enjoyed the fireworks (thanks to some Peltor Kids ear muffs.)
If you can, travel with other family members (grandparents, for example) who would LOVE to spend time with your baby while you and daddy get a few hours alone, too. You can also take advantage of Kids Night Out, if you are comfortable, for a nice dinner out. We used them when DS was 14 months and they were great!!!
I would plan your trip for when after your baby is sleeping through the night and you have mastered breast feeding, if that is your feeding method of choice. I will say it was soooooo much easier for us traveling when DS was 6 months vs 14 months. He napped much easier in the stroller, slept soundly though the night and most importantly, was not mobile! I wquld also shoot for Dec. It will be very moderate temps (although it can be cold!) and the lights for the holidays will certainly catch baby's attention. You can also take advantage of getting a few great photos with Santa in Epcot!
Good luck and congrats! I am adding some pics of him at his 6 month visit. Again, he doesn't remember it, but the memories for us are great and the pictures are my favorite!
DS on Small World
At
MVMCP
At Osborne Lights
With Tigger at CP....all the characters LOVED on him!