Since your application roughly approximates the cart trails at a golf course, 4 Ga will probably suffice, unless you're an efficiency nut like me.
I got mine from
scottyb at Carts Unlimited.
http://www.cartsunlimited.net/Batter..._Upgrades.html
As for lock nuts on the battery studs, use Stainless Steel.
Be careful about over-tightening. The stud is actually just a SS bolt with the hex-head embedded in the lead post.
Torque spec is only 95-105 in/lb (about 9 ft/lb).
I use a screw driver handle 1/4" drive, so I don't absentmindedly put too much muscle on them.