This. Exactly this. IRM 141's / 181's would definitely sell out at this point in time, but IRM probably have their rollouts planned 24 months ahead.
Using the A to test the water for price / quality / quantity, it would still mean finding time to do the research and development.
Not so sure about the C Class for three reasons.
Firstly, it's too similar an era to the A Class. With so much to cover both before and after the A, launching a loco from the same period may not make sense. It would be like doing a second run of A's.
Secondly, saying that the C would be easier because it's similar to the A etc doesn't sound like IRM's modeus operandi. They've shown that it's about quality, attention to detail, innovation value for money. A C Class would probably be built from the ground up, not using much, if anything, from the A Class.
And, third, an 071 or a Hunslett would go far better with the upcoming NIR coach packs.
A steamer seems likely at some stage, but with literally nothing RTR to go with it, I'd expect to see an older coach model before a steamer, something that overlaps steam and diesel.