DT used to be a good brand a few years back, they have good range and a wide level of stimulation levels also have a beep function and a vibrate function.
The way I was taught to use them is with the vibrate feature as a reward so the dog knows when its doing good too. Using it as a warning prior to a shock is not a good method. It will just create a collar shy dog.

So just as with being able to correct the dog at distance, you can "praise" at a distance.
Start close on a lead, blending the verbal , physical and electronic methods together before going to more distance.
Never use one in anger, you have to be very clinical about it.

Probably best left to an expert unless you have some experience with them or know someone that can show you.