I would rethink the sunshade. They are very impractical and I'm not sure how they work for the buses.
Rear baskets cannot stay on an ECV when taken on Disney buses. They must be removed. Not sure about the sunshade. And what I find interesting is that when I see someone with a sunshade, I see the shadow on the ground that the sunshade provides. About 90% of the time, the person using the ECV is not in that shadow. I think they are really only truly effective when the sun is at its highest point in the sky.
There are rental companies who do rent ECVs that will not fit on the Disney buses (not many but a few). That was what prompted Disney to paint the ECV squares at all the bus stops. If it fits in the square, it can fit on the bus. "Preferred" vendors will not rent ones that won't fit on the bus as that is a part of their contract.
Now with that said, there are some great nonpreferred vendors out there who don't carry those oversized ECVs. Walker is not a preferred vendor but they are one of the first companies to rent offsite ECVs. If they were so bad, they wouldn't have survived all these years.
My suggestion is to check the company's history with the BBB. Walker who is not preferred has an A+ rating.
Buena Vista (I believe they are a preferred vendor) has either an A or A+ as well. Preferred vendor Apple has an F. Apple tends to be cheaper than others, but cheaper isn't always better especially when it can cost you time or added stress when you are on vacation.
They will probably ask your height and weight to make sure the ECV model is size appropriate. The biggest decision will probably be whether you want a 3 wheel or 4 wheel model. Personally, I much prefer the 4 wheel model as I find them to be more stable. Some people feel the 3 wheel models are easier for turning. I find the turning radius on my 4 wheel model to be just as good as any 3 wheeled model. I also feel that I have more footrest room on my 4 wheeled model. I can stretch my legs out straight on mine but could never do that on any 3 wheeled model.
You shouldn't have any problems making it through a typical park day if you charge the battery every night. Also when sitting still turn the ECV off and this helps conserve the battery. Examples of times you should shut it off could include waiting for or watching a show or parade, waiting in a stagnant line (i.e. laugh floor comedy, tough to be a bug, tiki room, country bear jamboree, Hall of Presidents, etc... they load in batches, so you stay in the same place for several minutes at a time), while dining, watching the fireworks or nighttime shows, while sitting still conversing with others or even just people watching. Those types of situations are ideal for turning the power off and conserving the battery. But for all intents and purposes you should be good for power for typical park days.