Please, call it Motion Platform, not feedback

With this interface, from what you don't have some things, like
analog output. In general, you need to use a microcontroler,
as the interface between the computer and the platform will have
some delay for a PID controller.

The most expensive part should be the proportional directional hydraulic
valve, this valve can control the flow of the fluid precisely. You can try
controlling proportional manual valve with an electronic device, like
a motor, but I don't know if the cost is compensating, and some
other things.

You will need to detect the actuator position, and send it to the
PID controller ( that is feedback ).

Its not a simple thing, then you can start reading some things like:
- PID control loop
- Closed loop control
- Proportional hydraulics

You can try using a simple directional hydraulic valve, and switch it fast,
with variable duty cycle, but you need some other knowledge first.

Diego