You do not need the manual, you can download that. See if the cords are frayed or taped, are the decals nice? IF attachments are there, that is good, if not, they are easy to find. You can purchase cords and foot pedals separately if you need to.
You want to see the bobbin casing, ect to make sure it is not rusty. Look at the carrying case, does it look nice? The machine was probably stored in a closet somewhere. If it looks musty and gray, it will small bad, and so will the machine and you may find rust on the inside workings of the machine. The carrying case tells ALOT about how the machine was cared for.
Once you get your machine home, if there is some unpleasant odor, (mine was a little stinky) put a boxed bar of Irish Spring soap inside the carrying case and close it up. After a couple of weeks you may find the soap absorbed the nasty smell.
I have not done it yet, but I am planning on taking some old faashioned black shoe polish and shining that case right up! Works great on my boots and I am thinking it will make the case look even better.