What should I look for in a mattress to prevent bed sores & pressure points?

Mattress not crucial. Mattress is not crucial, as long as it's not too hard. It might be better if mattress is compatible with person's body shape. More important is to turn body around (changing position) every two hours, avoiding any specific (body) spots being against any object/hard substance for too long.
A floatation system. You should look for something that distributes support equally over the bodies surface.
Movement. Sustained pressure in any position will lead to pressure sores over boney promises. One of the best but most inconvenient beds ever designed was a kinair. This hand sand like particles that were blown around with air causing pressure points to change. They are difficult to get in and out of and may be difficult to keep clean with bedridden patients but the pressure points are constantly changing.