I have personally sat in the Kauai, Bella, and D'Amour. Nordic makes a basic, quality product, but they really cannot be compared to the Caldera, or even the Jacuzzi you listed. Besides a lower price, you're going to notice the different shell material, which costs less to manufacturer, but also is not as comfortable, in my opinion.
The Bella sits really shallow, even though its 38" deep. It has good power, but you sit very upright and I think if you put 2 people in it, the water may overflow. I don't remember anything that stood out to me about the D'Amour other than that I though it was a little under-powered, but also with all Nordic's, you can feel the jets against your back, even when filled and running, which I don't like.
I like the Kauai. It is 220V only, just an FYI. You have the ability to direct water flow where you can have full power to the lounger, full power to the two bench seats, or reduced, but equal power to all jets. When you have power to all jets, its a little underwhelming. I do like the therapy a lot in the Kauai though.
Never seen or sat in the Jacuzzi J215, so cannot comment on that hot tub.
If you have the local options, I would also suggest the Marquis V65L or Spirit, Hot Spring Jetsetter, Jacuzzi J-315, Sundance Dover or Capri, Bullfrog R5L / A5L, Dimension 1 Triad, and Artesian Santa Cruz. 110V vs 220V determines how large a pump can be in the hot tub, how fast it will heat and maintain its temperatures, and whether you can run the heater concurrently with the pumps. Hot Spring, Caldera, some Marquis and I believe Jacuzzi and Sundance can go up to 106, others 104. As for budget, that's up to you what you want to spend. You do get what you pay for and if $3k - $5k is where you want to be, then Nordic it is. If you can consider spending as much as $6k - $9k, then all others will be within your budget. I personally would want to try the model I am interested in. Small hot tubs are just that, small. The Martinique will feel much bigger than the Kauai is. Everything is condensed in small hot tubs. If you can't wet test, you need to at least dry test to see how you fit.