Get yourself a bunch of paper thin long sleeved cotton or linen/cotton shirts. Absolutely wear only once before you wash and iron.
Get yourself a bunch of very light weight linen, cotton or wool trouser in the typical light grays, taupe, tan, ivory etc. These should be unlined.
Very light weight wool socks and soft calf or suede shoes.
Absolutely no waistcoat. And a suit, jacket, or sports coat should be forbidden in 35-40 C weather. If any clients complain that you are...