to answer your question i done it twice iand i dont know if your any better off , unless they change it ..
My first car on motability back in 1983 i was 17 then you could buy cars using motability finance , wanted to keep a car after paying for it i didnt want to renew a car every three years , i think the criterior was car had to be under 3years old with i think max 30k miles .so my very first car a three year old white Datsun cherry with 30k on the clock. i paid the same amount of money every month my full mobility payment. but i also have to pay for servicing maintenance tiers insurance(omg the insurance was a lot ) i think i still got free road tax.
After the three years were up my car was done , so i got another car, same again bought it with motability finance three year old white vauxhall cavalier with 30k again i had to pay all the above again. i loved my cavalier,
Then i meet my wife had a child and my wife said your not putting our son in that death trap (my cavalier) , I want a new car , so i leased my first motability lease which was a Peugeot 205 and iv leased a car every three years or so since then.