basically when you sand down you use different grades of paper, they go from really coarse like P40's n P80's, down to really fine paper like P2500/P3000
you can also get 2 types of paper. wetordry paper.. which basically is for wet flatting, instead of rubbing it down dry n creating dust, you rub it down wet . wetflatting is mainly used for flatting primer n flatting new and old paintwork. dry sanding is used for repairs mainly where you gotta rub down bodyfiller.
you'll be using some P800, P1000 or P1200 wetordry paper to wet flat your spoiler, or a grey scotchbrite (scuff pad like a brillopad) if the paintwork is v good.
paint from halfords is about fiver a can if theyve got your colour in, if they have to colour match it its tenner a can.
your best to go to a proper paint supplier and ask them to colour match your car.
your gonna need 2 cans of paint n 2 cans of laquer for a spoiler. might get away with 1 can of colour but if you bugger up you got some more to do it again