Headed/headless is another consideration - two of my Seis are headless. All three were bought second-hand.
So, headed/headless, fretted/fretless, number of strings, body type, facing wood, core wood, neck woods as starters.
Then you have to start thinking about pickups and electronics. How many, what type, placement, any additional stuff like piezo pickups and 13-pin output.
Finally there's the little extras like LED fret markers.