Good luck with this build.
Anythingfish has great experience in this type of thing, as has John PTC. In fact Johns current tank is a scaled up version of what you want to build, but with acrylic windows instead of glass. Wizzin has also built somethin' similar with a wooden frame to hold his glass.
As mentioned earlier, use the search function on here to look for the projects these guys have built, you'll get mostly all the info you need. Have a look in the Articles section too.
I would insulate under your floor. It DOES make a big difference. All you would need is a polystyrene layer - 1/2" to 1" thick would do - then pour your floor slab on top. We use this for insulating concrete slab floors in houses and it works a treat.
It's not so much about locking the heat of your pond IN, but keeping cold seeping up from underneath OUT.
All the best with it, hope this helps.