First of all, it is perfectly natural to grieve the loss of your freedom and mobility, even if you only use the
ECV at WDW. I did, and others here have reported it as well. It's hard. The first time I had to use an ECV at WDW, I was there - in the Parks - and had to call a vendor to bring one out to me. I sat there and boo-hoo'ed over it. No kidding, it was ugly cry time. The poor CMs all kept trying to get me to go to First Aid, but I was *sad*, not sick!
But something funny-weird happened. Instead of everyone staring at me... no one really paid any attention to me. I was just another human on a scooter. No big deal. *And* instead of me being the reason we had to stop all the time so I could sit and rest... I was suddenly the one who could go go go all day, and do more and still keep going all evening, while everyone else was ready to go back to the hotel.
You *can* still savor everything about WDW - I am just like you, and I have found that we just have to do it a bit *differently* is all. First - and foremost - you are right about watching out for others. You are driving an open wheel vehicle, and sadly, most folks don't think about the physics of how scooters don't really stop on a dime. BUT... the thing I learned is that I only have to find a spot out of traffic, out of the way of the bipedals so no one walks into me, and I can sit and really look around for as long as I want. (Many of my favorite places to just sit and take it all is World Showcase.) Sometimes I will move my scooter just a few feet - just enough to change the view a little. I know WDW well enough now to know the places that I want to kind of focus on and so I will stay far to the right side, and go slowly, and find a spot that looks good, and then I will just park, and sit, and have a silly grin on my face because I am so happy to be at WDW.
Instead of worrying about how much it will cost, do the legwork right now, and call some of the vendors down there. See who has the best deal for you. That way, you know sooner rather than later exactly how it will impact your budget - and if nothing else, it lets you budget for that extra expense. Here's a bit of info for you: The WDW Parks scooters that are rented by Disney at the entrances to each Park cost $50/day. Typically, you can get rent for cheaper than that from outside vendors, so you may be able to trim that anticipated $100 for the Parks scooters down.
Some people may stare, it's true. As my friends above have said, some folks just don't have any home training, you know? There are rude people everywhere, not just at WDW, it's just that sometimes I think they stand out more at Disney World because most folks are so nice.

People like that come from a place of ignorance and fear. So, we ignore them. If they get in our face, then we give them a big ol' smile, and a heartfelt "Bless your heart" (which is the best southern shade you can throw!) and - most importantly - we go on, and we forget about them, because we are at Disney World, and we don't have time for that nonsense!
Buses are a great example of a time when you may catch someone who appears to be staring at you. There are those people who have never been to Disney World - or maybe they are from somewhere that there is no bus service, so just watching how the bus works in general is interesting to them. Then, along comes a human on a scooter, and the bus beeps, and barfs out a ramp, and then the scooter is driven right inside. LOL when you think about it, it's kinda crazy

So, they are staring at the spectacle of the bus and it's beeping and it's ramp, and whoever is on the scooter is secondary to the whole thing. Or, like me, they are so exhausted and overwhelmed, they have "zoned out" temporarily, and don't realize what they are even staring at!
If you have a cane (regular or folding) be sure to bring it, so that when you have to park and walk a bit, you will have that support. Hopefully, you will have good days, and the cane is just insurance. But better safe than sorry.
Overall, people are much more tolerant now of mobility devices and differently abled people at WDW. You *might* encounter someone who isn't as tolerant, but if you do, just remember that
we go on, and we forget about them, because we are at Disney World, and we don't have time for that nonsense!
Have a lovely trip - and let us know if you have any more questions about using an ECV at WDW.