Looks an interesting project. What period are you considering modelling - for the pre-grouping/early grouping period David Eveleigh has done a number of appropriate kits, covering the A class 4-4-0, the B 4-4-0T and is currently designing the C, D and Da classes. He also does kits for appropriate Great Northern and Midland carriages. Details of some of these can be seen on the 2mm Scale Association website, under Small Suppliers within the Products section.
Steve