Don't forget about boss 10..
I can't tell ya what the difference between S1 and S2 might be.
How ever. After getting my S1 boss 10 setup for dirt cheep (ie less then most manual S1 or S2's) it was dirt cheep because of being CNC I guess.
But either way if I were to do a conversion it after getting mine I'd say look for a boss machine. If nothing else you will already have something that contains all the movement ball screw type parts. Long as the machine is in good shape that should get you off to a head start in the conversion.
I was planning on converting the drives of mine over to something (since my other machines run Mach3) but people here convinced me to at least try the old drives and I've kept them. Kinda a bonus seeing I can run 2 machines at the same time and if converted I could only run 1..hehe
They can be querky but do work and at least mine works very well. Consider what I paid for it, it works extreemly well.
Do some times wish it had some manual handles though...