We've spent a lot of time looking at the different variants of Mk3 units and how much is common (and how much is annoyingly subtly different), so we are reasonably confident we can do at least the 320 (pre-refurb?), 321/4 and 321/9.
Come and talk to us about a 456 (either at TINGS or Warley) - we have a few ideas (though of course they were refurbed and lost their toilets).
As Ben said we'd rather get one lot right than compromise or try to offer too many variations making tooling either difficult or (more) expensive.
Cheers, Mike