You can rent an
ECV at the parks, but they can't leave the parks. That means you would have to walk around citywalk.
We just had to get an EVC on our last trip this summer. Granted we were staying onsite, so if you are staying offsite you might have to do something different. However, we just called the hotel and they booked an EVC for us from a company they use (apple scooters I think it was). When we got to the hotel, the EVC was waiting for us there. We even had a problem with it one morning where it was not charging. Called star services and they came and picked it up for us from the room and dropped off another for us in a matter of minutes.
As far as rides go, EVERY ride at the park can deal with wheelchairs of any kind. Some of them will take you to "backstage" areas to access an elevator, but everything has signs telling you where to go. If you are not in a wheelchair but can't manage stairs, ask the team member at the front of the ride (every ride has a team member at the entrance before you get in line) if the line has stairs and if so, you can use the wheelchair area to avoid them. Universal is really good when it comes to dealing with wheelchairs.
As far as where a bus will drop you off, I am not 100% sure. I think it is to the side of the Universal park (where their own busses drop you off if you don't take the boat I think) which means the only long walk will be getting to IOA. I could be wrong, so wait for somebody else to answer on that question.
Just google search wheelchair rentals near Orlando. There are a ton of companies to pick from if you are staying offsite and many will drop it off at the hotel for you for free. If you are staying onsite, give the hotel a call and they will be glad to help you.