It’s all about how much you can spend. There is nothing inherently wrong with any of those trials, in fact the only bad thing I’ve heard about one of them (The Devil) is that the frame gets torn up pretty easily.
Perhaps stay away just from the starter though.
If you have the extra money for a KH or an impact, then by all means I’d recommend just jumping past the medium level trials like nimbus and koxx, and progress on up to something even less likely to break.
If you don’t have the money for it though, then I think the nimbus trials would be your best bet, slightly heavier than any of the impact unicycles and the KH, but still quite good.
I think that Koxx-one is a pretty good deal. It looks like it’s in very good condition. The rim, hub, tire, cranks, and post are all good quality. The downside is that the seat base has a little flex to it, and the frame dents easily. So don’t get mad and throw it or something. I think it is a good place to start. It’s only a couple of upgrades away from being a really nice unicycle.
What kind of riding are you interested in? The uni is set up for trials – 140mm cranks for control and no rollos to bash your ankles on. That means it will work well for trials and street riding, but if you’re into flatland, you’ll have a hard time with rolls. There is not much room for your foot on the cranks. Same for the Nimbus unless you upgrade the cranks to Spirits and buy some rollos.