The mobility component of PIP is about £270pm, this is what your mother will get regardless of what the current car costs per month, you could very easily get a 3 year old car plus breakdown plus insurance etc for that amount of money, the only way you wouldn’t be able to would be if you were looking at a brand new car and if all you need is transport to help look after your mother then you don’t need a brand new car and could easily get 2nd hand with her PIP money.