I see that the SX already got sold. My recommendation was going to be to build the Auto-X anyway and here is why:
1. You wanted some suspension to make the ride easier on your back. I think the 200x suspension would be better than the SX.
2. WATER! You want to be able to ride in and around water. The SX rear end is not suitable for water riding UNLESS you take some precautions. (You have to stay on top of it, check axle lube often, check seals, make sure things are sealed up well, etc)
I think the Auto-X will fit your requirements really nicely. The SX is an absolute must-own machine as well, if you ever get the chance, pick one up since they are great. But for your immediate needs, you won't be disappointed with the Auto-X.
There are lots of threads on the Auto-X conversion. The couple of items you need to figure out (other than what you are dealing with on the motor mount on the head) are the foot brake and the shifter. On my conversion using a 200S motor, I just needed to put a few washers in between the left footpeg mount and the frame to provide clearance for the shifter. The brake pedal was cobbled together from the 200S donor brake pedal and the 200x parts. There are some really slick ways to rig up the foot brake (mine not included). Searches should turn up some good options for you.
Good Luck, you're gonna love the Auto-X.
- Frank
1984 200ES Big Red
1985 350X (x2)
1986 350X
1986 250SX
1984 Auto-X
1984 ATC70
1985 ATC70