If there is literally no minimum for undecorated models, then could thought be given to offering all three types of unit (320, 321/4, 321/9)?
You see, from what I can work out, of those models you are looking at, the 321/9 seems to be the closest fit to a 322 (just two windows are different I think) and on my layout I'd like some of those in addition to a 321/4 (preferably Silverlink if I'm honest) and a 'Metro' 321/9. An undecorated model (or potentially a primed white version) would suit me perfectly for the 322s. A painted example would also work I guess, but might be a bit more effort.
I'm sure that those who want a Saltire 320 could manage with some transfers and could probably 'block out' the unnecessary window(s) as required.