Well, naturally Nikon would recommend one of their flashes.
And the 800 is an excellent workhorse flash. I've seen them in very good condition in the 150-200 range.
You don't absolutely need TTL. If you want to learn how to use flash, I'd recommend just getting a good manual flash and, if you don't already have one, a meter with flash capability. Alternatively, if you have a dslr, you can use it as a Polaroid to dial in the flash settings.
Use cheap film. It isn't that difficult and you'll learn quickly.
Not that TTL is such a bad thing, but I find it boring. If you want to take better flash photos, you have to get the flash off the camera.