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.

Been using Sportdog collars for aversion training for quite a few years now and have been v happy with a pair of them.  Yes I am one of those who had to trial it on myself before I could allow it on my dogs so I have a healthy respect for using the shock feature.  I've found though that even with the dial on 8 I'm now only experiencing a tingle where once 4 was as high as I'd go.  To be expected with age and for quite a few years regularly use at Kiwi aversion days.  Wondering now if anyone has ever replaced a battery or knows anywhere that does or if I'm better off buying new gear price and reliability wise...???