We had this problem until recently. There is no easy solution unfortunately. If you need all 7 seats in use the whole time with a large wheelchair then only something like the Hyundai i800 will do. However, if the wheelchair breaks down and folds then a galaxy would maybe help.
We have 3 children who are slightly older and so only need 2 booster seats with backs on and nothing for eldest. We also have a wheelchair that does not fold up and has a head rest and postural support on it. At the moment we drive a Peugeot 5008 7 seater but we don't use the 2 boot seats. This easily fits DS's wheelchair (with headrest taken off). However does not fit it with the back seats up.
We have just ordered (on motability) a Honda CR-V. This has a big enough boot to fit the wheelchair and is large enough to fit 3 kids on the backseat, but only because of the smaller car seats. The only other options for us on motability, were the Galaxy, s-max or a big van like i800. All of these have a down payment at the maximum end.
We have always leased/financed our own cars before now but finally decided it was silly not to use motability as we would have a much smaller car and use it a lot less if it wasn't because of Ds and his wheelchair. No finance option came even close to the motability deal.
Do what we did, check out suitable options on the motability site and then go round to those car dealerships with all car seats and wheelchairs and stick them all in to see if they fit. No dealership had any issue with us doing that, even though the wheelchair was particularly muddy in the lovely clean showroom cars!!
Sorry for the spiel, I hope you understood my waffling!