To be honest i would not reccomend a 125 if your not that quick if that makes sense! lol
125's need to be ridden very hard to get anything from them,but are very rewarding when ridden right
It,s swings and roundabouts to be honest a 250 4 stroke would be the best bet as with a 4 stroke they have lots more bottom end grunt to pull you out of any trouble (ie wrong gear or a bit tierd)where as a 125 you need to be in the right gear all the time or they don,t go basically!
A 250 2stroke has a similar affect as a 250 4t as they have the grunt to pull you out of sticky situation's but when they come on song into the power band they are gone!!!
Be-aware though a 250 four stroke mx bike will have a very close ratio gear box ,small radiators so will boil very quickly if not kept cool,And the main ingredient after 1.5hrs of hard muddy slog.........The magic button!! ELECTRIC START!!