Bachmann brought out a newly tooled Jubilee a few years back, covers the short firebox locos but may be possible to fit an old long Firefox body on the new dcc friendly chassis?
One thing on your plan that troubles me, if you curved the Dumfries siding round between the main line and the branch, you wouldn't need any gradients, your boards would be simpler, and your locos will pull longer trains. Just a thought.