If you care about hardware in the portable PC, get a later Dolch 61-64 but not a 65 they aren't mechanical. If you care about the keyboard only, the 60 is best since it has a PS/2 port on the side, making the cable detachable and it works with any PS/2 cable. the others require a custom cable be built but that's not hard thanks to some work by geekhackers contributing the pinouts on here.
One of the models has a trackpoint of sorts in the top right corner, I've never used that one. It might have been a variation of the 64c.