One thing I will strongly advise you is, keep the heat low, don't be tempted to rush this as overheating or trying to do it quickly will leave you with cracks, etc, all over the place. I'm assuming the floors have been screeded, what about upstairs, is it hollowcore? Next is cleaning out the house and completely cleaning down the walls and ceilings especially as you intend to spray walls and ceilings. Personally, I find spraying the whole lot a great way to get that 1st coat on, makes a wonderful job of coving and ceilings if you're going for white, but remember, the place needs to be dry and clean, I can't stress that enough. Finally, lots of people think that painting is simple, easy to do and that they can save themselves thousands by doing it themselves. Yes, they can save themselves lots of money, but any decent tradesman will know a DIY job from a mile away and i'll guarantee you they'll pay a hell of a lot in the long run to get it sorted. A good painter is a good tradesman, they don't come cheap. Anyone who's good at their job will cost and good one's will also give you a quotation.
What about door frames and architrave, a stairs, are presses fitted, etc? No, I don't think the price you've been given is excessive, does it include vat?