Unless it looks like it was recovered from the Titanic, buy it.. everything is fixable.. the motors are available. the grates are available. and as long as all the parts are there that is a very reasonable price..
Things to try or check, but not deal-killers.. plug it in,, the distribution fan should run, and the stoker motor should run, the stoker mechanism should move back and forth. Look at the grate, it should not have cracks burnt in the side-walls of the grate.. new ones are available, but it is an expense...doors open and close,, you wil need to replace the rope gaskets on the doors, the ashpan [s] intact or rusted out. The glass in the door cracked and discolored [doesn't matter even if cracked,, just all the pieces there].
Hope this helps..