I'd say starting with an unweathered model would be best. Renumber it, and then weather it. Done right, the weathering would look more realistic than the weathering on the MM models, and you would also have control over how much weathering you wanted. As you know, there are a number of people on this site who can do this work to a high standard. If you wished to use one of them to weather the loco, Marks Models post to Ireland free of charge, so you could arrange with Marks to have the loco posted directly to whoever was going to do the work. I'd say if you paid for the loco before the end of the sale tonight, Marks would probably wait a few days before posting it, giving you time to sort everything out....