Cutting-in Decorating

This ultimately depends on the size of your room and how much woodwork you have! if it is just skirting then you will need an undercoat and 1litre of satinwood.

This depends on what your doors are made of and what sort of finish you would like. there is not much difference between using a brush and roller on a door. i use a brush because oil based undercoats should be worked into the wood to give the best protection and first layer, a roller doesn't quite achieve this. going with the grains of the wood is easier with a brush.
a small 4 inch roller with a short pile sleeve is generally used for window sills or to get a flat finish, not so important on wood doors