Valve seals not too difficult to replace. They usually do not cause too much oil use like rings. Smoke on startup will not be burned off by cats until they get hot.
Valve seals usually will give a puff of blue smoke when foot is lifted from gas pedal.
Something is strange about that car! Blue smoke out of the tailpipe at 120k miles is totally unusual. Cats that aren't plugged but yet don't get hot enough to burn a little of oil vapor. Maybe it is just water vapor.