Drainage is important, and you know that. I would use a water shield as an outer barrier. 2" poly isocyanulate inside that against 4" cinder/concrete block. I used 1/2" radiant tubing inside that and former the inside with mortar. These were mud bath tubs at a spa. 30yrs later they are still going. I am not sure what you would want to use for an internal finish. It would be nice to have a smooth finish. Seems it wouldn't matter in a mud tub, but would in a bathtub. Also, I guess it being outside you will need a cover as well as a cap for the walls.

Gayle, ours is a Hot Springs Spa, Vanguard model. I don't know how it compares to Sundance in size or jets, but if I recall correctly, it cost somewhere around $8000 installed (in 2004). No matter which brand you choose, make sure you can actually try out the spa with water in it. When we were looking at spas, we found there was a huge difference between just looking at or sitting in an empty spa and actually feeling how the jets work. Yes, it's a little odd being in the middle of the store relaxing in a spa, but I think it's worth it. The corner seats in our spa have neck jets. I just love them, although they do kind of spray water up into your face sometimes. And if you have neck jets, you need to keep the spa filled very high (all jets must be submerged). Which means that if you have more than the usual number of people in the spa, it may overflow. Not a big problem, but definitely something to think about. Here is a link that might be useful: Vanguard spa

Honestly I would call a local pool builder. You are basically wanting a indoor spa. 12 on 12 #3 rebar, with 4 bars at the top and a little thicker 12", and 6 to 8" of concrete/shotcrete in the walls. You can insulate under it, but remember that is a lot of weight, and insulation could crush over time causing it to move. Not really necessary. Running heat strips in the pour sounds like a reasonable idea. Unless water table is high or something don't see sealing exterior. Seal interior well and probably tile it, just like you would do any wet location. Again, call a PB, they could hook you up.

Can extra stored electricity be used to run an electric heater with enough heat for a hot tub? This is separate from the pool solar system. We are being told gas is the best option but that will be much pricier for us as we have excess electricity. Thank you for your help!!