I want to buy the flat I rent (privately from landlord). How do I come up with a price to offer?

I need to get a realistic price on work that needs doing it (a survey maybe? Happy to pay for a full one if I have to?!) And a market valuation on it, would rather not have to pretend I'm the seller to some estate agent, want a completely impartial view. Also, what percentage to estate agents take off, so I can reckon this into a private offer, what are the legalities of making a private offer?? Sorry, lot of questions in one lump!