Wheechairs

Does anyone know about wheelchairs? My mother has asked me to find out prices for 'a bog standard non motorised wheelchair'. The situation is that having lost a huge amount of weight rapidly through undiagnosed diabetes, muscle wastage means that her legs sometimes collapse beneath her without warning and she falls and hurts herself. Although physio makes things a bit better, she wants a wheelchair because she doesn't feel safe. Her arms are also not strong enough for sticks.

Finding out the basics isn't so hard, a google search brings up the Help the Aged site and Wheelchairs-R-Us(!). What I'd like to know is what makes a good wheelchair? Any review sites? She'd be pushed rather than propelling herself. In the meantime, Venice in four weeks' time arrrgggh. Finding a chair that can cope better with cobbles would be good. bigger wheels? But lightweight and transportable also necessary.

PS
Do not go on crash diets, kiddies. :-(