When I've made a offer like this in the past I've tried to be open and honest. I let people know that I use to be a realtor and I know the market. I ask them if they are willing to sell their property and ask what is their price. Ask if you could look around the property. Don't let them know that you already have.After they tell me I then tell them what I'm willing to pay and why? (show them comps).
Once you start getting closer to a price that you like ask them to let you inspect the property have people come over and look at what it will cost to fix the property. Once you get a due diligence period (contract) the process can be shorter, well if your paying with cash. It makes the seller feel like your more serious and it may make them come down with the price and feel better about it. You don't want them to second guess their price and then change their minds.
I do try to be very honest with them about how bad the home is and how much it will cost to fix it. I do a lot of the work if not all of the work myself it never cost me the price that I say it will but I never know if I will need to call in someone to actually do a job for me , so it could actually cost me the price.
Realize that if you don't give a person the price that they want, someone people will get nasty, but remember your goals and that you are trying to make money and not lose money and they are trying to do the same thing. Try to get as close as you can to the price that you are willing to give for the property. If you don't get it trust me there will be others.
I don't know how much experience or money that you have but their are a few ways of buying homes. You can do rent to own, bank finance, use your 401k, hardmoney, family memebers, your own cash from savings or bonds.