Hiya JDY!
Right, there's a lot of info to had here but I'll start with a couple of answers for you.
If you are going to use buses and the underground a lot then it's best to get a weekly travel card each instead of pay-as-you-go on a daily basis.
For food and incidentals i'd say use a similar budget to what you'd have on vacation.
Do you have any more info about the trip, i.e. where you are staying etc?
BTW - fire away with any questions about London here or by PM - I've lived, worked and played in London all my life.