Is it possible to install air conditioning to a 1991 toyota hilux from scratch and will it be expensive

Yes, And Yes unless you go to a junk yard for your parts. But it will be a headache to do yourself. If you have the money to do it get a dealer to do it. Regular shops are good at AC but not installing the whole system. Running the ac lines and installing the evaporator and dryer in a car that never had ac can be a hard project

Yep, what he said, along with associated wiring, relays, AC coil inside the heater ducting, etc, etc.