What are the main considerations for design of air conditioning system?

First of all u need to calculate room volume and ACPH and calculate CFM,

Main consideration of HVAC Designing  should be to meet the target of the owner and end user, achieve his target in low cost and give a durable solution.

But don't compromise with local and international standards, rules and regulations.

Your question is very general, I am afraid this is the right answer you are looking for or not.