That's a little high. That boat in good condition would sell for about $12,000 around here. But if it is in excellent condition and was keep in a dry stack, it could be worth the asking price.
Everything depends on the condition. That engine is reliable but uses a lot of fuel. Low hours are not necessarily a good thing on an older engine. Take it for a test ride and see if it makes the right rpm at full throttle (5300-5500 rpm) and idles smoothly. Do a compression test and drain a little lube from the lower unit to see if there is any water present. Visually inspect everything under the cowl looking for leak, bad wires, hoses, and corrosion.
1999 Gradys used rot resistant Greenwood XL in the transom and stingers. But it still should not be wet. Check out any screws that you find in the transom and be sure that they are bedded properly with sealant. I'm talking about transducer mounts and trim tabs here. It's best to have a professional surveyor test for moisture with a meter, but you can check for rot yourself with a ball hammer. Tap the transom all over and listen for a hollow sound or any change in the sound as you move around. Visually inspect as much of the stringers near the transom. Be sure that the glass over them is still intact. The stingers are not as likely to have a problem as the transom.
The floor also contains wood that can get soft on an older Grady. Walk over the entire cockpit floor to be sure that it is solid. Only the access ports should sag when you step on them. Pop off the access ports in the floor and look at the condition of the connections and hoses at the filler and fuel sending unit.
Turn on all of the electronics, horn, and gauges. Make a list of everything that doesn't work. Find out the cost of any repairs. That list is how you can successfully negotiate the price down without insulting the seller.