Joey, This mower new sold for about $15,600. I looked in the offical guide for outdoor power equipment and find that currently this mower is worth about $6,000 ~ $7,000 depending on condition.
The power unit should have the s/n plat somewhere on the chassis. The engine should have the s/n close to the injection pump area.
Hope that helps with your quest
Mission: Constant Never-ending Improvement