sounds like valve seals,you 1st start up and it smokes then after 15-20 min in stops,thats because the rings have expanded.
seen a few of mx3s for sale like this and checked at the garage,aparantly a expensive job and you will need a new head gasket as the head has to be removed and the bottom end does,well thats what ive been told by the garage after taking 2 with a view to buy after check up.
good job i went garage 1st ay