Salary Sacrifice will come out of your salary before tax and deductions are considered BUT the remaining income must be more than the equivalent of earning minimum wage.
As long as the deal offered includes everything Motability does and that the mileage is sufficient, it sounds like a much better deal. Remember, Motability takes 13 payments a year, so any payment around the same monthly rate saves you around £300 a year.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark