You might want to check out an article on the West Marine site about chosing the right anchor. I have a Danforth anchor with 2m of chain and 40m of rope.
How to select the right anchor
I have a similar sized RIB with a Mercury 60 2-stroke. I can get from Poole harbour to Durdle Door and back (40nm) on a tank of fuel (23litres/5gallons), crusing at about 30knots. I guess that works out at about 3.75gallons/hour.
I would expect much better fuel economy from a 4-stroke. I had a boat on the river thames with a 9.9HP 4-stroke. It ran for about 7 hours a day for 5 days on only 3 gallons of petrol.