We need more info from you cocker. Type of house? How many stories? How many rooms? How many external walls? How many rooms? Doubleglazing? Have you good loft and cavity insulation? Are you thinking of zones, upstairs and downstairs for example? How good is your water pressure? Are you using your existing heating system, that's rads and pipework? Where are you thinking of putting the boiler?
And so on. And that's before we start asking environmental questions so we can advise on what type of oil tank you need, where you should put it and what base it needs.
I'm not trying to put you off cocker, it's just there's a lot of considerations, it's not just bang a boiler in.