As above, food, water and petrol money. Particularly if its for friends.
I did some time as a decorators mate so if you need any pointers.
Paint choice is crucial, depending on the surface to be painted and the finish required it may pay to get a better 'one coat' paint. They are more expensive but you will use less in the long run and will probably only have to go over heavily soiled areas with a second coat. Dulux one coat or Crown solo are both good, but I would lean towards crown.
Unfinished surfaces (plaster, brickwork) will need 2 coats regardless. Wallpaper is best removed but can be painted over and sometimes leaves a nice texture once overpainted. Lifting edges can be glued down with wood glue and wiped afterwards with a damp cloth. Any bits of varnish will have to be scraped/sanded off surfaces to be painted or the paint will not stick and will flake off when dry.
Definately get a tube or 2 of decorators caulk and a dispensing gun and a tub of polyfilla. Helps with little gaps and cracks and for sealing around skirting boards, and door frames to give a more finished look.
And of course plenty of masking tape unless you are good at cutting in, particularly if there is any carpet anywhere and dont forget to do the ceilings first
