This is a long hard slog. List all the components. Go to the manuals and get the heat dissipation for on load and off load conditions. Look at what will be on simultaneously. Some manufacturers will given heat dissipation at different loads. You will be shocked at VSDs. The better VSDs will have through mount so the heat sink goes into the bus bar compartment. Soft starters go off line with by pass contractors, so unless it does frequent starts, you can neglect that. Preparation plates need .tp be perforated to allow the air flow. IEC 60439-1 and the South African SANS allow 130 degrees Celcius as bus bar temperatures. This is very high if you have toair condition the room for Africa conditions. When you add the cost of air-conditioning, energy to run them, maintenance and the people to fix it and petrol to get there, I calculated that the once off price to run at 70 is a working option.
Don't forget to add the energy and heat for stop and start lamps and any HMI units. There is also heat dissipation in the stop push button if you really get technical. Good luck.