Buying a water damaged property

I'm thinking of buying a property that has been left empty with a leaky roof. its been repossessed so the heating and everything is out of it and I can see some of the ceilings have come down. Am I better to attempt project manage the renovatin myself and getting tradesmen in as and when needed, or does it generally work out cheaper to approach a building firm to do all of the hard work in a oner. It needs a roof repair, a new boiler fitted, the electrics checked out - possible rewire, new plasterboard put up, skimmed etc. Theres water on the inside of the windows and mould everywhere. note that I haven't bought the property yet but I'd like an idea of what it would cost to do it up so I can figure out an offer price and see whether its worth my while