I did a few years in this when I left school but turned to motor trade after. R134a is the most common and user friendly and can buy it quite easily. I'm sure R12 is more serious older gas to be phased out soon so R134a or R401a would be its replacement. Are you getting a HVAC engineer in or doing to yourself. Ive had a few burns from gas it freezing but bloody hurts after. Might be able to pass you on to my mate who still in industry for advice