Both here and on the
Kiteboarding subreddit, I usually advocate for a 4-line fixed-bridle foil powerkite on handles as a trainer. For someone just starting out, it gives all the control (but no depower) at the lowest cost; and doesn't require a harness.
...but since you probably have a bunch of kite gear already, you can leverage that; the bars and lines specifically. To echo some of what others have said: If you have a small (5m-ish) SLEI, or can find one for cheap; that could work if you're in the water, and can fill the role of a storm kite for you. I wouldn't want to put an inflatable in the hands of an abject beginner on land. That's a repair waiting to happen.
If you'd prefer to do the introduction on land (most do), a depower foil like clubkite mentions is your best bet. FS has the Viron, HQ has the Apex 4. The cheap alternative is the
Pansh Blaze II. I'll be getting mine this week, so I haven't tried one yet; but I've heard it works pretty well. These can also be used for traction activites on land: buggying, all-terrain boarding, snow kiting.
Just remember that the safety system on whatever bar you're using probably won't work with a foil--they are usually flagged on the rear lines instead of the fronts (as with most inflatables). You could make your own safety system, but the easiest thing to do if you have a bar with "Oh Shit Handles" is just to clip the kiteleash into one of those.
While I respect the simplicity of 2-3 line trainers; I think they do a piss-poor job of teaching anything other than a very basic introduction to the wind window. And I think it's why people tend to write-off trainers entirely, or quickly abandon them after hitting the water. If that's all you want out of a trainer, then fine; but if you want to learn any sort of actual kite control, you need four lines.
