Deckle and Gorton were the king's of those.
The specifics depend on what you want to do. Some of the pantographs were just intended for engraving, such as markings in dies and molds, while others were capable of real stock removal. There's also hydraulic tracers, but they are vastly overkill for anything a knifemaker would ever do.
There is a bit of a toss up in size as well. Small ones obviously take up less space, big ones are obviously more capable.
Large ones are also generally (substantially) cheaper as not many people want one taking up space. Same reason a little atlas or southbend shaper (glorified toys) will sell for a couple thousand, and a big Cincinnati (incredibly capable machine) would be $100 on the high end.
So my advice would be to figure out how much space you have, and exactly what you want to do with the machine. That will determine specifics to look for.
And then the standard machine tool buying practices apply, checking for slop and wear, missing or broken parts, signs of abuse, ect