Thanks for the help so far everyone.
Just one last question: If I go for 5x10 wheels and 165/70 tyres, are these going to be wider than the body of the car? (I have no arches)
They probably will protrude beyond the body if you've got disc brakes, you might get away with it if you're on drums.
It's all dependant on wheel offset.
I had 5x10 compomotive tf'f on my archless mini in the 80's, they didn't protrude at the time as I was still on drum brakes but as soon as I swapped to discs the wheels protruded and I used to get pulled by the old bill on a regular basis and warned about the wheels sticking out. Still took quite a few tugs before I filled arches though.