for issue
1) Visit a reputed AC shop, IE basheer AC at Tariq Road / KBW road. See if everything is compatible or why the AC is taking extra ordinary load.
2) Since the AC is taking load, Probably the compressor is swaped or mix matched or probably its dead that is why it is creating issues.
3) Brake light comes on after you start the car? Or is it on even after?
Brake switch can be faulty, its is a under rs. 500 job.
A basic tune up, Throttle body service, new spark plugs and a catalytic convertor clean up should help you fix your basic issues.
The feeling hard/ taking load can be a bad compressor/ jammed pulley.
The AC being tripped is the first sign of the cars tempreture going up.
You mentioned heat up on stand still, This means your cars radiator is chocked. Get it serviced or get it replaced with a brand new one, Shouldn't cost more then 7 to 8 thousand. avoid getting anything from scrap.