The interior of 69, as above, on the DCDR, will actually be a completely different layout to ex-WLWR 900.
69 has a brake compartment in the middle, with a saloon either side. It was, of course, never like this in real life, nor did any railway that I am aware of have a coach with a guard's compartment between two first class open compartments, so the rebuild of 69 is quite unique; in operational days it was only ever a full brake (and one of only 2 or 3 six-wheel vehicles to receive black'n'tan).
No. 900 had a single saloon, with a toilet at the non-windowed end, and a sort of "drawing room" compartment with two facing seats in the window end, as shown in the diagram with the pink background.
The diagram of 322 / 324 / 325 below shows the layout of about the only three first saloons that existed (might have been a handful more), but these had a toilet in the middle and bench seating. An all-first saloon without a toilet would not have been likely. I am unaware of any such in saloon form.
Bottom line is that there will be nothing at all comparable to the rebuild of 69 - but the finished article will be an absolute masterpiece nonetheless, and with double doors invaluable for wheelchairs and children's buggies. Let us hope it becomes a regular performer as a service vehicle instead of special days out only!
It was, of course, not at all unusual for railway companies back in the day to rebuild vehicles with completely different configurations to what they originally had. The MGWR's 1M is a good case in point - variously a first class diner - or a third class seated coach! And with six wheel bogies, then four wheel ones.
Second image shows the trio of first saloons with fixed bench seats and the last one shows 69 as it was throughout its railway service.