Well, I've had my Vespin for awhile, but it cost nowhere near $800! Shop around a bit. You can get an idea by checking what Terry sells it for, but he won't ship - you have to pick it up! Obviously, not likely possible for you.
If you don't want to deal with the extra gap behind the toilet, a 14" rough-in toilet really is your best choice, and the Totos with the Unifit will project into the room less than most (if not all).
The bolt down lid (the top of the tank) is designed for places where they don't want people messing with the internal parts of the toilet, like a public restroom so someone can't take it off and break it, for example.
You probably won't know if a new toilet will cover the old spot without some very careful measurements and luck.