Nathan, how much scratch building have you done ? It is a long painful process to end up with a good likeness of what you are seeking. You start with hundreds of photos and measurements. Photos with tape measures in them, etc, etc. I sweat just thinking about it.
My vote is find the closest model you can find and using your photos paint it to match your prototype. You can add any custom details you wish, such as under frame details and perhaps some of the items you can see ,and photograph, through the open doors. I have received permission in the past from the crew of some of the Locomotives to climb all over locos and tenders to make detail photos. Also found it helps to have a typed up release of liability form you sign and hand them. Norfolk Southern ran a steam shop at Norris Yard in Birmingham where N&W 611, 1218, Southern 4501 and T&P 610 received major repairs or were tore down to the frame for total rebuilds and I was allowed for years to roam the shop making photos of the locos and progress of the rebuild. BTW; 765 as well as C&O 2716 Kanawha / Berkshire, the CP 2839 Royal Hudson and SP 4449 have been in that shop. Was a sad day here when they closed the steam shop around 96. I think the last time they ran 611 out of Bham was December of 95. I chased it to Chattanooga and got 20 or so good shots. J