I've been told the general rule is that once your temp has been normal for 24 hours, you should no longer be contagious and be fit to go out in public if you feel like it.
The bigger issue is going to be fatigue/stamina and that is something only you can answer, and probably not until closer to the trip. Fortunately, WCs are usually pretty available. I would recommend is that you research and decide which off-site company you want to rent from if necessary (Offsite companies are usually cheaper than renting from Disney day by day). I usually use Apple as they are low priced and a preferred provider, which means they can leave the chair at the resort for you to pick up when you get there, and you can leave it with the bell services for pickup when you leave. But there are stickies at the DISabillities forum that list the providers that have gotten good feedback from DISers.
Then, when it gets down to the wire, if you are not sure whether you need it or not, you can try going without. If you wear out too quick, you can rent one in the park that day, and call your selected provider to have one dropped off at your resort for the next day. You don't have to sit in it all the time. You can park it in stroller parking and walk for a while, or you can put your park bag or backpack in the chair so you don't have to carry it and walk using the chair for support. Remember, though, some of the queues are very long -- Soaring is 1/4 mile in and another 1/4 mile back out.
If the heat and humidity knock you out too much, you might want to consider going to a first aid station to lay down for a bit. They have comfy cots in dark air-conditioned spaces (some in curtained off cubbies, some in individual rooms. My friend sometimes has to go lay down for 30 min or so until her migraine meds kick in, and there's never been a problem. They would rather have you spend 30 min in the First Aid station, then get back out there spending money, than having to go all the way back to your resort and possibly take the whole afternoon off.
