I've ended up putting together the third set of Super Barn castings for the model of 103 for a client, so I seem to be committed to building a batch of 3 rather than 2 2's.
It's gone together as smoothly as the others, although I had a moment of brain fade when it became apparent I had forgotten to mark and drill the holes in the floor for the bogie pivot bolts. Whoops!
Fortunately the chassis isn't too far advanced so I was still able to measure and mark the position easily and hold it firmly enough in my hand to drill through a couple of mm of styrene without dislodging any details.
So the plan is to add the bits to bring it up to the same point as the other two, which means a rib along the top of the roof, some little bracket bits along the false framework and some corridor connections, and then I'll make some more progress on the first pair.
I have to choose between finishing off the underframes, doing something about the roof skins or making some masters for interior seats and tables which I would be able to make casts of.