Have you thought about something like an F20 116i? Got the same engine as the mini cooper. I know you had the revious 1 series but the f20 is light years ahead of it in every way including boot space for your sons wheelchair.
They are doing them for around £280 with no deposit and obviously discounts are to be had on that if you ask nicely. I've just got 11% off on a 125. I looked at the options from Ford, Volvo and even Kia and nearly all worked out more expensive or within £20 of the baby beemer.