I would be more concerned about what is going on inside the motor then how it looks outside. With it out you can't do a proper compression test. Looks like a non Pontiac q-jet on top?

Has it ever been rebuilt?
Was it bored?
Was the crank turned?
Cast piston and rods?
557 casting block?
how many reliefes in the piston?
What cam is in it?
How many miles does it have?

If its not a 557 block and never rebuilt it would still make a good foundation for a rebuild in the future if the price is right. I think I would only pay 300 or less since its yellow:)