Did the inspector tell you what the pressure is? If not You should first get an accurate pressure reading before installing a Pressure Reducing Valve ( PRV ) . I recommend to call up your local water district and ask what your normal pressure on the street is. You can also buy a pressure guage at your local HD and hook it up to your hose bib NEXT to your main house shut off. However It's a 50/50 shot if the hose bib is prior to the House shut off / and or prior to the house PRV. So your guessing at best by this way.
Just install it just prior to your Main Shut off to the irrigation system. Hopefully your Anti-siphon valve is a back flow assembly. This is the easier way to test the pressure by using the test cocks of the backflow assembly. Care to share a picture?
Don't skimp on quality-
Use nothing less then a then a brass PRV. I Recommend the bullet proof
1" Wilkins Zurn Pressure Reducing Valve . You can shop around for a better price of course and your local HD/ Lowes should have these
Also , I agree with Hookoo regarding the RPZ.
If you only have a
Anti-siphon valve then I recommend to take the sucker out and replace it with
Reduced Pressure Back Flow Assembly . This should act as the main shut off to your irrigation system as well as provide ample back flow protection.