I have done solo several times with just my ONE dd since she had turned 4. I can't imagine doing WDW with her when she was an infant as she was a very high maintenance baby not to mention with 2...solo. She was always very good, now she gets a little tough because she's 7 1/2 and getting that mind of her own thing going. When she was younger we'd go to the park(s) for a few hrs and then come back after lunch and a nap, now we do try to come back and relax as she doesn't nap much anymore.
I don't think you're crazy. If you have "easy" kids that won't leave your side (of course it would be hard for the 5 mth old to do that) you should be ok. I would try to choose a "slower" non-peak time to go and just have your mind set to do a few things at whichever parks you select to go to. Get started as early as you can so you have time to rest yourself. And, try to pick a time of year when it's not unbearably hot or rainy. (tough to do in FL but...)
I would definitely go with a double stroller of some sort, one that's somewhat lightweight since you're the only one carrying it. You may be better off driving to the parks if you have a car because it will be difficult folding it up to get on a bus especially with an infant and a little one and a diaper bag. Keep in mind you will need to go through security before entering the park.
If you don't have a car, definitely stay on site. At the MK resorts you would be able to wheel on the stroller onto the monorail to MK as at the Epcot resorts you would be able to wheel on your stroller onto the boats. At the MK resorts you can also use the monorail to Epcot. Unless you have a car I'd avoid AK as the only way there is by bus. And, if you plan to go to MK the only way there from the Epcot resorts is by busy unless you take the boat and walk through Epcot to the front and take the 2 monorails (that would be VERY time consuming).
Good lucK!! I wouldn't do it, but it is doable if you are a patient, low key person
