I think Geoff's 'frets' are steel not brass – so a pair of Xuron snips will quickly be destroyed.
More likely to need a small pair of bolt or music wire cutters, or a hacksaw / fretsaw – either will need some small files to remove the 'pip' that is left, clean up the laser-cut faces and remove sharp corners..
I would prime all the steel parts before assembly.
Choice of top coats and when to apply may depend on how detailed the kit is (does it have strapping internal to the body too?), and how faithful to the original you want it to be. Generally, timber-bodied wagons were not painted inside.
Andy