I'd lean towards the 763, the 773 is vertical lift i think, it is good for loading trucks as it has vertical lift, so the bucket is further forward to load trucks, but it has more pivot points to wear.I would definitely skip the 843 due to its age, in its hay day that would be a great option. With that said, what do you plan on doing with the machine? The 763 is a great machine for digging and the 773 is more of a load and carry machine. The 773 can lift higher than the 763, other than that they are basically the same machine and can do the same work. Good Luck.