When Sweeter cures all that is left is epoxy as all of the solvents evaporate so attaching the window is just like epoxy. Sweetwater does have haz mat shipping charges and I'm guessing will end up costing a lot.
I think I mentioned the EPDM tape in a previous reply. People have used it to seal off the folds but I don't have any experience with this. I have never heard of overlapping it to seal a tank. Would be great it if would work. I just glanced at the price for the 6" tape online and at $2.60 per lineal foot that's VERY expensive.
This build is a bit more difficult, no doubt. But learning to work with fiberglass and epoxy or fiberglass and polyester is a valuable skill in this hobby.
And you might be right, for now just build a basic plywood pond and have the ramps and land area be completely separate. If you really want to keep it simple don't include a window for now. If the tank holds water long term a window could be added later.
The one things you do have going for you is water depth. The shallower the less likely to leak for a first project. And since Spotted Turtles don't do well with deep water it's not like you have temptation to build them the biggest, deepest tank you can.