If it has the Series 1 Kohler, it's worth about the $550 he's asking. The early Series 1 had a bad lube design and if operated on slopes would starve for oil and self destruct. If it's a series II or Onan, then it's worth in decent condition, minimal rust, no leaks, no smoke, no bad noises, around $1000 or more including the mower deck. Worth more if equiped with the optional rear integral (sleeve) hitch, rear PTO, or any other attachments. This was the top of the 300 series in the late 1970's, but the early models suffered from engine problems as mentioned above. It's the last of the closed rear frame models and was replaced by the legenday 318, a total redesign. If you can furnish a serial number, I can give you more info, like year made and original engine installed.