If you look around the valve it should give you the model number and sometimes its replacement model.
The valve is in two parts the brass part and the controller box which sits on the top and houses the atuator.
I think that the price of the complete unit is almost the same as if you only bought the controller box. So I would buy the complete unit.
Theres no need to drain if only changing the top controller box.
Remove by unscrewing the two machine screws that secure the controller to the brass body and pull it off completely. Leaving the wiring for later. You now have just the brass part with stainless steel plate on top with a metal machined pin sticking through a rubber gasket
Unscrew and take the controller from off the brass body of the brand new unit and put into place onto the existing unit. You'll see that existing brass body unit has a pin sticking up from a stainless steel plate. This pin has a machined straight edges so that the controller must align with them and slot down onto the pin. The new Honeywell unit comes with full fitting instructions.
Regarding the rewiring (always make sure the electricity is off and you've got the fuse in your pocket.)
I always replace only 1 wire at a time i.e. at the connection box I only take the existing brown wire out and put the new brown wire in its place. Then the same with blue etc., etc., till all wired up exactly as before.
If you still get screeching them some s**t has found its way into the brass body and is stopping the movement of the rubber ball inside. At which point a drain down is reguired, making sure the valve is in the manual or Mid position to drain down.
Onced drained remove valve from pipe work and find out what's causing the bockage, if no blockage can be seen, replace existing brass body with the new one that you already have.
Fill up again with water and Inhibitor. Valve to be in mid postion to fill up.