Only collar to buy is the Garmin 550 pro. Everything else is rubbish and won’t last the distance.

Every comment so far has been based on aversion training only. You need to be clear in your goals from the outset? Are you just going to use it for aversion, example dog chases sheep and gets burned for it. Kiwi aversion training is the same thing.
Or do you want to learn how to train a dog properly with the collar? Correct e collar use is the most humane way you can train a dog. It is also the quickest and most effective It is no different to the use of any other form of pressure with the main emphasis in this type of training system being teaching the dog how to understand and turn any pressure off.
Before you go embarking on this journey you need to have your handling/training  skills finely honed. Your ability to teach, timing, understanding the dogs intent, knowing when to back off and simplify, knowing when to push forward are all key skills you will need. Understanding when a dog has made a mistake and when it has deliberately disobeyed are also really important.
If you don’t have these key skills sets then you need to get that sorted first.

No one has yet talked about properly e collar conditioning the dog. This is a 2-3 week lesson where the dog is taught to understand the pressure the collar produces and how to appropriately respond to that pressure. It is completed using known commands and pressure that the dog already understands, example choke chain. It is a complex lesson that needs tuition and guidance the first couple of times it is taught.

The training stimulation levels I use on my dogs I can use on myself on my own neck. The idea that the dog is being shocked are incorrect.

The last thing I would say is you will need to get the old human emotion of anger/frustration under control first. If you get angry or frustrated during training now then that needs to be sorted first before you strap an e collar on a dog.