Fred Bebar

I'm shopping for a used 2017-18 Silverado LTZ, and would like to have GPS without OnStar. I see upgrading is fairly simple - there are many videos on YouTube which should make it simple. Not for ME mind you, but for my BFF mechanic who can probably knock it out in under an hour. There are a number of places offering the gear for $800-$1000, but there are also a lot of sellers on eBay offering them for as little as $200. My question is this: are there different variants of the box? If so, how do I determine whether a box is a match for my vehicle? And is there some subsequent programming required and if so, where do I find a technician who can do that? Any help I can get would be appreciated.