<div class="ubbcode-block"><div class="ubbcode-header">Quote:]The seller says that the valve guides are new, which could point to rings.[/QUOTE]
Not necessarily. New valve guides doesn't rule out smoke from valves on startup....if they didn't properly install (or forgot to install) new valve seals, you'll get that little blue puff on startup.
& if it were rings, it probably wouldn't stop when the engine got hot as the oil thins out & would continue to sneak past bad rings - it just wouldn't be so visible.....get it hot & stand behind the car while a friend races the engine...look, feel at the tailpipe, & smell for oily smoke.....if its rings, there'll be a telltale presence.
Before I did anything, however, I'd do both a wet & a dry compression check to see if there's a problem & where its located.
Plus, what's the oil pressure both cold & hot?
Edited: I believe the car's telling you something different than the owner (
https://www.theautoist.com/buying_an_MG1.htm)