That's not how the Motability Scheme works so this person who is waiting for a tribunal result and planning to buy that particular Mercedes is going to to be sorely disappointed. They certainly won't be able to get it through Motability. You can't buy a car through the Scheme and you can't contract-hire whatever you fancy. The option that used to exist to buy a car on Hire Purchase via the Scheme was scrapped some time ago too. The up-front payment you now have to pay for a more expensive contract hire car isn't a deposit either - its a one-off, sunk payment that you don't get back even though you don't ever own the car.
WAVs aside (where the provisions are slightly different) you can now only sign up for a 3 year contract hire deal from Motability. All you get is the use (hire) of a car for 3 years. And you can only sign up for one of the cars that is actually offered on the Scheme - those are relatively limited. Cars that are not included in the Motability list are not available to you. You never own the car - you only have the use of it for 3 years and then have to hand it back at the end of the 3 year period. There are very limited circumstances where the lease can be extended for a further year.
The makes and models available are limited to cars with a maximum, new, dealer list price of approx £30k. Motability renegotiate with the car manufacturers every quarter so the car list, prices, and costs change every quarter. A car of that rough value (£30k) requires you to pay around £3000 as an upfront payment (which you don't get back) and then your weekly PIP allowance in full for the 3 years. The scheme used to contain a much, much bigger selection of considerably more expensive cars with massive up front payments but the coalition government put paid to that - Motability cut the selection to cars massively to only those with a maximum one-off up-front payment of £1000. That has gradually crept back up to the much more useful max £3000 up-front payment that applies now.
Personally I think that you should be able to contract hire whatever car you want via Motability if you are prepared, and able, to afford the up-front payment that it demands. People all have very different needs and desires when it comes to cars and lifestyles. If you are working in a good job, have hobbies requiring a big car or have plenty of cash from savings or a compensation award for example, why shouldn't you have a big, fancy, expensive or specialist car? Its makes no financial difference to Motability - the individual is paying up-front for the additional costs of an expensive car. Not everyone who receives PIP has state benefits as their only income or has a job with a low income and there's no reason the Scheme can't cater for everyone regardless of income and personal circumstances.
Motability, the DWP and the government should be doing a much better job of the PR so that the public are no longer under this ridiculous illusion that people on PIP get free cars and that would go a long way to removing the ranting, right-wing, Daily Mail reader attitudes about folk getting fancy free cars which is, of course, based on pure myth.