This will very much depend upon what you need from the car. If you need a hoist & other adaptions then it may make sense to use Motability, wheras if all you need is a small manual car, then you’d be better off buying that at an appreciable discount from a dealer, or indeed leasing if you like your cars shiny new.
If you are the type who buys a car & runs it for 20 years then definitely do that. When my disability got bad enough that I could no longer drive the car I bought in 1997 I gave it to my son & he’s still running it, at £13,500 new that works out at £587 per year plus things like servicing & consumables.