whats needs for solenoid control?

what do i need to buy to get this project to work. i have a pressurized oil line that feeds a piston. the piston controls a v-belts pulley diameter. the pulley drives a belt driven air compressor. i want to be able to set the pressure needed with a dial and have a solenoid valve move the piston until the pressure in the air compressor matches the dialed amount. this is a low pressure high cfm compressor. the valve will only need to flow a very small amount of oil, but will need to be able to make fine adjustments to the piston. it must be a 12v dc system and i will monitor the air pressure with a analoge pressure sensor. would it be cheaper to use a proportioning 3 way valve or two on/off valves. do i use a pic or a plc. thanks.

Use a PID Controller

PID Temperature Controller. AKA, Process Controller.
Analog input and output.
Input is Pressure Transducer.
Output is Proportioning valve.
Dial is Controller setpoint.