85, 800, 171, 461, most likely in my opinion. J15s would probably still sell well too but so soon after oo works, I'm not sure.
Maebh (and the 800s) would be the obvious choice but she has sat in Cultra for the last 60 years, whereas the RPSI locos have been out and about, and all the major RPSI locos have decent class numbers (apart from 461).
V class hit most of the right marks, big loco, nice livery, decent working career, and still working in preservation (good use for those RPSI coaches) and 5 of them. Very little GNRI rtr rolling stock available atm would be one minus for them, whereas a southern loco has the six-wheelers and the irm laminates coming through the pipework.
For GSR 4-4-0s, surely the go-to would be the 301s. Still think it is unlikely (at least for a good while) that we'll see a southern 4-4-0 in rtr, unless oo works decides to have a lash at it.