I have 20+ years in the hot tub business and I would not attempt this project without hiring help. I have seen dozens of pools and tubs that were eventually filled in due to major structural issues or code violations that were impractical to make compliant. Most of these were built by 'professionals' although unlicensed or incompetent contractors are often part of the problem.
Here in the U.S. you would need several special permits to build any structure that amounts to a small swimming pool. You are digging a hole, lining it with concrete, filling it with water, connecting it to electricity and then jumping in the water. You really want to get the water + electricity part right before you stick your toe in the water.
The project would require several inspections along the way such as ensuring the rebar structure is properly grounded and bonded before pouring the concrete.
It might also require health department inspection to ensure the water turnover rate is sufficient and will definitely need an electrical inspection to ensure everything in or even near the bodies of water is properly wired and grounded.
The in-floor heater you propose will not work. Most in-ground tubs use gas heaters but in-ground tubs that use electric heaters typically use at least a 11,500 Watt heater. The in-floor heater you propose only generates 293 Watts.
Nearly all electric hot tub heaters use an element in direct contact with the water (like the water heater in a house) for maximum heating efficiency. Heating the floor will also heat the ground underneath and the Earth is the biggest heat-sink on the planet
Any in-floor heating system (no matter how many watts) will be highly inefficient.
Nothing in the description of your proposed floor heater suggests it is safe for your intended use. It would need to be CSA approved for use in a potentially wet environment
. Concrete is porous to begin with and small cracks are inevitable which is why even the rebar is inspected for proper grounding and bonding before pouring concrete. Keep in mind the rebar should never carry current and your in-floor heater does.
Your 30 Amp breaker will most likely be inadequate. The pump you propose using draws 16.4 Amps on high speed. A typical 5.5 kW heater pulls 20 Amps. Even most self-contained, 'portable' tubs these days require 40, 50 or 60 Amps to operate and most of these tubs are 8' x 8' and fully insulated.
I could write several pages on why building your own swim spa is a bad idea but will summarize with this: Several manufacturers of portable spas have offered 'swim spas' over the years. Some of these 'swim spas' have had totally inadequate performance and these companies (theoretically) hired hydraulic engineers to design the systems. I personally know of several lawsuits filed over inadequate swim spa performance.
You definitely get an "A" for ambition
but I think you'll be much better off looking on craigslist for used, portable hot tubs. There is no shortage of them for sale in this economy (even swim spas) and a used tub will give you plenty of DIY time.