I daily drove a 300gd without a/c for 5 years in Alabama. The 460 and early 463 Gs have a nice fresh air intake that does a good job bringing in a lot of air. Power windows help too if you can find a truck with them.
You should know that the a/c on 460 and early 463 trucks is not what anyone would consider “super-cooling”. The system is something you would find in 1970s-era vehicles. If a/c is really important, I would suggest finding someone with an equipped vehicle to let you make an extended test run in the month of August.
1990 250GD 461.401
2000 G500 463.241
2006 G500 463.249