You know, flying with an ECV is quite do-able. My personal one is very lightweight and so flight crews, gate agents, WDW bus drivers all love it!
Seriously, the first trip flying with an ECV (or other medically necessary equipment) is the hardest one. Once you learn how your favorite airline will handle the ECV (and procedures do vary from airline to airline, *and* airport to airport) then you know what to expect. He will be able to use the lift on the DME buses from and to MCO as well.
Even if I am not flying to WDW, I can't make it through most airports unassisted now, so I have no choice but to bring along my personal ECV, or deal with airport services... and I value my independence too much; I can't stand being pushed around in a wheelchair by someone else. If I want to go grab a cup of coffee, or run to the bathroom while we wait for the flight to board, I can. Additionally, I know my own equipment, and I am far more comfortable using my own equipment. It's like driving your car - after you have driven your car for years, it feels weird to hop in to a rental and drive a whole different vehicle. You have to adjust to everything being just a little bit different. For some people, that's just *exciting*. For me, it's stressful!
I use my personal scooter at home far more than I thought I would. Not just when shopping, but for museums, street fairs, concerts, events of all kinds. I have regained my independence; I no longer have to worry about things like "how close can I park?" or "how long will this last?" because I have my personal mobility device with me. Please understand - I don't want to have to use it (no one who has to use wheels to get around *wants* to) but I view it as a tool to get things done. It's a tool that lets me go on and live my life the way I want to, to the best of my ability.
A lot depends on how much he has budgeted to spend, and what model ECV he is looking at. I purposely chose a brand and model that was created to be sturdy but light, folds down compactly, and has long battery life because when I travel those are the features that I value the most. My particular model of ECV looks "weird" to a lot of people:
View attachment 217891
I have had people ask me "where is the rest of your scooter?"

but because I don't use the seat back, I can drop the handlebars, and lift it in and out of my personal car (a Kia Soul) by myself, and if I need to store it, I can fold it down to the size of a golf bag. It doesn't have some of the obvious comfort features that a rental model does (like armrests, or a large seat back) but it does only weigh 35 pounds, and I can "double stack" my batteries and go all day and all night without stopping to recharge!
At the end of the day, it's all about his comfort level. The only downside I can see (aside from getting through airports before you get to WDW) is that if he is talking about purchasing a used rental-fleet scooter, it will have a finite amount of use left; additionally, unless they put a brand-new battery in it (which can cost anywhere from $100 upwards) he may be very unhappy with battery life. Batteries are the number one items to cause problems for ECV users. But the kind of wear and tear that a rental ECV gets (especially one that has been in use at WDW where it could easily average 10 miles per day, every day for it's entire life) will affect every component of the ECV. It is possible that he could buy a used rental ECV, and in less than 3 years spend more money fixing things than he would have spent to buy new ECV. I just want you to be aware that could happen. ECVs are a big investment for most of us, and I'm sure you both want to maximize your value!