Hi Paul,
While I can't help you with your request for interior photos (I know of none) I will tell you that the late Harry Percival wrote briefly of the interior of the Sheepscot station. I only remember his mention of a penny candy vending machine within that structure, not much else. Not having worked with the museums' archives since 2015, I'm not aware of any photos that may have since surfaced.
In my own collection of On2 models, I have the Sheepscot station and freight shed kits as produced by Sandy River Car Shops. I also recall seeing a model of the Headtide water tank that was built by a member from New Jersey and that kit was on display in the gift shop (at Sheepscot) several years ago.
Hopefully a reader can help you with your request, I'll just say "Welcome" and good luck on your project.