Other than spoke count, is there a strength ratings for wheels?
Unfortunately, it is less exact process than you hope for. Most important thing would be to locate reputable wheel builder who will build wheels for you. It can be an advantage if he is local to you. If there are some guys you know that went on the large tour of tenths of thousands of km, ask them who built their wheels.

Wheel builder would spec the wheel components according to your weight and riding style, from components he is accustomed to build. He would probably spec something along these:


rims that can take at least 35 mm tires
heavy duty double walled rims (Mavic A319 or A719 are often used but so are many others)
36 of 2.0:1.8:2.0 spokes