Welcome to AH Tony!
When it comes to plains game, and kudu and gemsbok in particular, there are almost too many choices in Namibia and South Africa. I'd stay away from Botswana because I don't support the closure of hunting there, but that's just me.
What this means is that you need to decide what else is important to you. Things such as:
1. Is it important that the animals be native to the area?
2. Do you want free range or will you accept fences?
3. If you accept fences, how big an area should it be at a minimum?
4. How important are other factors such as the type and style of camp?
It's by asking yourself questions such as these, and others, that you will start to whittle the number of outfitters down to something reasonably manageable. Once you've done that, you can get lots of advice here on the good, the bad and the ugly, but you can also talk to those outfitters who are sponsors of this site. While there is no formal vetting process (that I'm aware of), they are vetted in the court of public opinion, and a bad apple wouldn't last long.
Good luck, enjoy the site, and welcome to the addiction.