Of the kites you mention, only the Hydra can be used in the water. That said, water capability in a trainer isn't as important as you might think, and it's hard to justify the higher price. I can't speak to the relative build quality of these kites, but in general foils are pretty robust as they don't have bladders to pop.
While HQ makes a quality product, I think they and the entire industry as a whole do a disservice by promoting 2-3 line trainers with bars as an entry point to kiteboarding. I'll try to explain why as succinctly as possible. First, foils come in two different design schemes: Fixed bridle, and 'depower'.
Depower foils are flown on 4-5 lines with a bar and require the use of a harness. They have a fancy bridle system which allows the angle of attack (AoA) and profile of the kite to be changed dynamically in flight (in a similar fashion to most supported leading edge inflatable (SLEI) kites these days). Suffice to say, these are more expensive and not very popular in the 'trainer' market, because they really are full-fledged kites and the price reflects that. The Viron icebird recommended is of this type. They would make an ideal trainer but for the price, and the additional cost of a harness.
Fixed bridle foils are flown on 2-4 lines with a wide variety of control schemes, everything from straps to handles to bars. Because the AoA and profile of the kite is 'locked in', these kites are considerably cheaper. To realize further savings, most companies selling 'trainers' have taken the simplicity to it's logical extent: Fixed bridle, 2 lines, on a bar so that it looks like it bears some relation to an actual kiteboarding kite. A step up in complexity and price is a 3-line foil, which is really just the same as a 2 lines, with the 3rd attached to the trailing edge exclusively for relaunch and landing. The Rush Pro and the Hydra you're looking at are of this type.
I'm of the firm belief that 2-3 line foils on bars are rather silly and almost useless, and it's why people tend to sell them or let them collect dust after getting a 'real' kite. The reason is that they offer very little in terms of control because the power and the steering of the kite are all mixed together on the two flying lines. All they're really good for is a basic introduction to the wind window, and learning how to steer a kite with a bar--which are good things to learn, but you'd hope a 'trainer' could give you more. Further, I think they give beginners a bad impression of how to control a kite; they demand a bit of muscle to make them turn, when what's needed with a 'real' kite is finesse.
The middle road is a 4-line fixed bridle foil: Two front lines mainly for power delivery and 2 brake lines on the trailing edge to aid steering. The distinction between power and steering allows much greater control of the kite, and opens up more opportunities for learning. Because of their relatively low cost and good control, 4-line fixed bridle foils remain the dominant platform in the buggying and kiteskiing communities.
So, if 4-line fixed bridle foils are so much better than 2-3 line foils, why aren't they more commonly sold as kiteboarding trainers? Basically, it's because they don't fly particularly well on a regular bar; they usually require at least a little steering input on the front lines, which is why they're predominantly flown on handles. There are systems which make it possible to fly 4-line fixed bridle foils on a bar, like the Ozone Turbo Bar:
...or the fancy bridle on the Scout that you're looking at:
But since these systems don't offer any additional control and come at a premium, they remain something of a niche product.
I'm of the belief that a 4-line fixed bridle foil on handles represents the best combination of good control for learning, at a low price for the aspiring kiteboarder. You don't need a harness at the outset, but you can add one into the mix later as the next step in your progression. The ability to stall the kite, and learning how to recover it will put you *way* ahead when it comes time for your first lesson in the water. Finally, it will be great for snow kiting on windier days.
I got my first 4-line foil *after* I was already a competent upwind rider. Nonetheless, I got this little 3.5m foil for about $100 used, and it improved my kite flying skills dramatically:
Here's a picture of a friend flying it in about 15 knots of wind with a harness and strop line between the handles. He's probably about 170 lbs. and it has the power to take him for a bit of a drag when he puts it through the power zone:
So, enough power to make you respect the wind and the kite; but not so much that it can kill you unless you do something phenomenally stupid--on purpose.
Some people seem to think that learning on handles can complicate the transition to a bar on a depower kite. This hasn't been my experience in the least, and even if it were true, I'd contend that it couldn't possibly be worse than the ham-fisting a 2-3 line trainer promotes.
There are great deals to be had on Ebay for 4-line foils with handles (specifically from a seller called '4mkites' from 'china, china' with whom I have no affiliation). You'd want something with a lower aspect (shorter and fatter looking) and not too large, I wouldn't go for anything over 4 metres--my chubby 3.5m foil stays in the sky with little trouble in around 7 knots of wind. Depending on your size and the wind strength, fixed bridle foils start getting dangerous between 3.5-5.0m; by which I mean 'enough power to lift you off the ground, but not enough area to let your down gently.' Stay away from things like Flexifoil Blades, or Pansh Aces, those are high-aspect (long and skinny) performance-oriented kites; if you do a search for 'power kite accident' on YouTube, those models seem to come-up most often.
My reddit comment history
has a bunch of posts in more depth on this topic. I'm happy to answer any more questions you might have.