Sidney:
I don't think the Deltics pulled freight unless they were doing running in after shopping.
They would likely have a consist of whatever was the modern rolling stock, although ther could be a smattering (or more) of earlier stock. What colour is the loco? For the blue and grey era, you could have a mix of mark I and II coaches, possibly III. Once a month you could run a blue and grey painted Gresley or Thompson buffet car or full brake.
If you go for Maroon era, drop the Mark III and add a few more LNER diners, maybe a first, in extremis, an articulated restaurant car.
I don't think anyone could fault you with a full train of Mark II or Mark I. Train length of 10-14 cars.
(I'll check some old Railway magazines if you give me a typical year.)