Are you sure you haven't been smelling the petrol fumes?!!!!
In all seriousness, no one cal advise you with such little information.
4 stroke, 2 stroke, new gen 2 stroke will play a part. You then need to think about the conditions, you could use 1/3 more fuel coming back if the sea kicks up.
You need to spend some time inshore to get to know the engine you are yet to buy. One of the members with a 225 had an issue with something or other 25 miles out, on the way back he was burning so much fuel than usual he nearly ran out. He had no idea until almost too late.
Not sure what you are meaning re. pumps on tanks. Wait till you get an engine, the tanks are cheap enough to buy new / newish so you can be sure they are not contaminated.
Just get to know the rig, then worry about heading out!
FYI, I have a light rig with a 50 4 stroke, I usually go out with a 25 and a 12 litre tank.
Charlie A has a relatively heavy Shetland with a new gen 2 stroke 75 and often carries 50+ litres.
(4.4 litres to gallon btw!)
Just take it easy get to know the boats reliability and quirks inshore, the, as Jim said, venture further with a buddy boat.
Rob