It sounds like you are asking about buying a scooter, not just renting one for WDW, correct? I am not really familiar with how insurance works, but have you talked to them and to a Mobility supplier to know what your insurance will cover? They may only allow certain ones so you may not have much choice.
You will need one that is able to accommodate higher weights so be sure you pay attention to that. If you get one that lists 300 as the top end of what it can hold then you will have a less performance than one that is able to handle weights higher than that. Speed and battery life will be affected, so be sure that you are looking for ones with the higher capacity.
You want to look at battery life-- how long can they go on one charge? For around the town errands it isn't as big of a deal but for WDW you want to be sure that it will last the entire day.
Also how are the batteries charged? For my scooter the battery comes off and plugs into the charger separately so I just bring the battery in and leave the scooter in my van. For many you have to plug the charger directly into the body of the
ECV so you would need to plug it in while it is still on the lift or get it down every time and move it to where you can charge it. If you have a garage that will fit your vehicle and the lift then that it will be easier to just plug it in on the carrier, but if you will have to park outside then that may be more difficult, particularly in bad weather.
Different models have different speeds. While a higher top speed may seem cool, think about if you really need that. When would you actually need/want to go 10mph in it?

And how is the speed controlled, by a dial or electronic buttons? I personally prefer the dial because I feel it is easier and more responsive. I had rented one with electronic keys (not sure how to describe it exactly) and hated having to keep pushing up or down every time I needed to change speeds, especially when I needed to do so quickly. But that is just personal preference.
Most important is to try if at all possible to test drive before you buy. I went in set on getting one thing and when I sat in it realized it was very uncomfortable for my needs. This is a huge investment so be sure that it is comfortable and can accommodate all of your needs. You may want to look for one where the seat can swivel sideways to make it easier to get in and out. Think about if you want armrests or not. Check to see how much the tiller (the control piece) moves back and forth and how it is adjusted. Some are not easy to adjust quickly.
This is all I can think of right now that we considered before purchasing. Maybe others will have some more ideas.

Good luck! I really am happy with mine and the amount of independence it gives me.