Hi Sarah. Welcome to the forum. Pretty much everyone on this forum uses the Motability scheme and are therefore quite biased towards it (in a positive way). However, for every person who is a Motability customer there are two others who are eligible to join the scheme but choose not to, either because they source their own car (new or used) or use their benefits for other things, which is quite telling.
If you have a desire to have a fully expensed (except fuel) new car every 3 years then for most people you won’t find a cheaper way of doing so than Motability. However, only you can answer if the cost (£10k in sacrificed PIP over the lease plus Advanced Payment plus the cost of any options you want) is worth it.
As for timing, if you decide to join the scheme I would usually advise against ordering until you have passed your driving test. It could take you 6 months or more until you pass your test, all the time you are paying for a car that you cannot use (you would want to use a professional instructor and they use their own dual control cars). That being said, lead times for new cars are ridiculously long at present and 6 months is not uncommon.
Finally, you will find this forum to be a great source of information, including on choice of cars. I would have to be honest and say that Suzuki cars aren’t the best and the Motability scheme is unique in that you can sometimes get premium cars for the same advance payment as budget ones. Given that you sacrifice the same £10k in benefits whether you chose a Suzuki or a Mercedes, the cheaper cars tend to be the poorest value on the scheme. Obviously it does depend on what, if any, Advance Payment you can afford.