Congrats! Try custom function 01 to turn on your viewfinder info. Probably a good idea to reset all functions to default and set them for what you prefer.
Custom function 01 leaves on the viewfinder info during the exposure only. To the best of my knowledge, ISO setting info is only available on the body's top display window only. Generally speaking though, ISO is something that one doesn't normally monkey around with as much as aperture and shutter speed adjustments which do show up in the viewfinder info display. ISO is normally something you set once for a shooting session based on what your lighting conditions are and need to compensate via ISO adjustment for that. I do agree though that it would be nice if that info was displayed in the viewfinder but I it's not and life goes on!
Resetting the camera to the factory defaults is definitely a good idea when buying used though.