Proto 1000 is the only real choice.
The Athearn one is junk and too short - I have one.
The RDC's were very popular for connecting service and off-peak locals.
THe New Haven had one of the largest fleets of RDC's, including the RDC-1 (all coach), RDC-2 (coach/baggage), RDC-3 (coach, RPO/Baggage), and RDC-4 (all baggage/RPO).
The reasoning was to use the RDC's to feed passengers from the dozens and dozens of New Haven branchlines to the mainlines for mainline full service train connections.
They were extremely reliable - New Haven RDC's lasted until the mid 1980's in Amtrak service in NE.
Even today there are several operational NHRR examples including some in ALASKA.
They were powered by two diesel engines underneath.
It is prototypical to run 1single unit all the way to 8 MUed units which I've seen in NHRR books.
I've ridden in a restored NH one and it was quite nice - seemed like a regular coach with a bit of engine drone noise.
One problem though was grade crossings - before FRA mandated cab cars and SPV's have the same crush strengths, the cab of an RDC was where you wouldn't want to be in a collision.
These moved along at normal passenger train speeds too.