
Welcome to disABILITIES from me too.
Personally, if I was in your shoes, I'd want to start chemo right away. I'd want to be on vacation envisioning the cancer cells being driven away by all the WDW pixie dust.
But, if you are thinking about postponing your chemo, the doctor can tell you what the consequences might be. Sometimes it doesn't matter if it is delayed; sometimes it matters a lot. Your doctor can tell you which category you belong in and that might help you feel better about beginnig the treatment. A lot of times, the doctor's nurse is the one who does the education about what side effects you might expect from treatment, so I'd probably start with the nurse.
If you have had 2 treatments before you go, you should have some idea of how you may react before you go to WDW (but keep in mind that the chemo may have sort of a cumulative effect and you might feel more effects as you go on).
An
ecv is a good idea because it would help you conserve your strength for more fun things than just getting around. If you are feeling tired and want to lie down for a while, there is a First Aid station at each park where you can do that. The staff are nice, the place is dim and quiet and you can lie down on a cot in an individual cubicle.
A Guest Assistance Card (We call it a GAC to avoid typing so much) might also help, as Figaro suggested. The GAC is a way to let CMs know what sorts of extra assistance you might need because of an invisible disability (whether it is a permanent disability or a temporary one). You request one at Guest Services in any of the parks. You just explain what your needs are (which may be a place to wait out of the sun and heat) and the CM will try to give you a card with appropriate types of things to assist you.
This link has a lot of information about touring with disabilities. It also has a good explanation of the GAC.

Hope things go well for you.