Don't worry about the tires, they are easy to switch, focus on the carbs.
My '77 is set up almost eactly the same, Vetter SS and Bates bags. I paid $950 18 months ago. It ran, but poorly. Knowing now what I do about GL's and cleaning and adjusting the carbs, I might have offered $750 even though the mileage was low.
$1,000 - $1,500 is a very fair price assuming the bike runs well (that means starts easy and idles clean) with no obvious sign of burning oil, leaking water pumps, or blown head head gaskets.
Head over to
http://ngwclub.com/forum. You will find tons of info there on what to look for in a GL.