The question I have is "What engine are you putting in there?"

I agree with Minn that basic standard servos will be fine, as long as you aren't going fast.

I always wind up using whatever is around in the shop on air valves, very little force required. And I wouldn't use digital there because I wouldn't want to get the setup a bit off and eat the battery.

I've become a fan of the less expensive digital servos like the Hitec 5465. For $25 each, they really are nice. And depending on the pushrod setup and the way you fly, you can tell the difference. But for most of my planes in that size, I'm using Hitec 425's for most flight surfaces.

Note that flaps often require more torque, but not always. I'm using Hitec 322's on the flaps on my 72" Mosquito which is going to be a bit heavier and faster than your Mustang. I have one of these on each side, and it's plenty. If I was using a single servo, I'd be using something bigger though.

For rudder, elevator, and aileron, I usually just want something that centers well because a plane that doesn't hold a trim drives me nuts. That's one reason I've been moving towards digital, but frankly good pushrods and surface hinging matters as much or more.