Utilities are impossible to answer without knowing how far you have to go. For instance, Oncor allows a certain amount of money or a certain distance whichever gets you further. You can probably expect to pay somewhere between $9-11 a foot all in on running a water line from the meter to where you are building, but who knows how far you'll have to run the water to get to your property...it could be right in front, it could be across the street (you'll have to pay for the burrowing) or you could have to extend the water line yourself.
On the septic, I'm not so sure that I would want one installed. If is already installed, you are going to be limited to what it was designed for. For instance, someone might have put one in for a one bedroom/one bath cabin. If you build any larger, you'd have to redo the whole system.
The ag exemption is another issue. Usually, there'll already be one in place. If so, work with the owner/seller on keeping it. For instance, if they are running cattle, let them continue to run cattle, or hay, or row crops or wildlife exemption, etc. It's much easier to do that than it is to get one yourself. You are looking at a minimum of three years and lots of proof to get the county to agree to it if you are starting from scratch.