take the slide off and look and the frame rails. if there are scalloped cutouts, its what is called a Mud Rail. This was done to allow mud and dirt fall out and not hinder the slide action. however all it did was weaken the slide rails. Models with these cutouts have been know to split or crack along the rail. If it has this cutouts I would just look closely for cracks in the rail on either side.
its believed that they stopped doing this in 1987, but some believe it was done in 1988. I don't think the "mud rail" versions are worth less, just something to look for.
I wouldn't pay more that $550 depending on number of mags included. any less than 3 mags, I would go down. but depending on wear condition id say the range is $350-550. as Runco said, tade-ins have sold for $325