i brought my bjorn, but didn't use it once. don't know why...maybe i was too lazy to put it on?! anyways, my dd is 8 months now and we were there just last week. i carried her in line for toy story mania for about 25 minutes...not fun! she's about 20lbs. that was the one day i wish i hadn't left the carrier in the hotel room. i'm sure other mothers will have other recommendations for carriers.
we did MM on thursday so the lines were pretty quick...so no big deal holding her in line. for nemo friday morning, we had the stroller in line for as long as we could, then i think i had to carry her for 15 minutes or so. we were lucky that morning because astroblasters and jungle cruise were walk-ons that morning after nemo.
as for the stroller, i'd bring my own. if the baby was older, then maybe you could do the umbrella stroller. if you have a disney visa, stroller rental at DL is free. otherwise, i think it's about $10-$12/day. we brought out double stroller. yes, it a bit big, but i was surprised to see the number of double strollers in the park! i swear, every other stroller i saw was a double stroller!