As an owner of the old Lima made GWR railcar from the late 70s / early 80s I am glad that Hornby has dusted off the old tooling and hope that the new model has an improved motor.
Despite my enthusiasm for railcars #19-38, I second Alexander's request for the original curvier railcar design. #5-18 are probably the best candidates.
My preference is to go even one better. I think that the GWR steam railmotor and autotrailer (like #93 and #92 as being rebuilt by the Great Western Society) would make a great trainpack.
The GWR had 99 steam railmotors and they served from the turn of the century to the mid-1930s on branch lines and Plymouth suburban commuter services. After all, there were only 35 Gresley A4s built and their working lifespan was about the same. (I know the A4s look fantastic and all have names and different liveries and hold the speed record, but they aren't Great Western!)