The biggest problem as a lot have already noted would be scale as Transformers are all over the place. Core would be the most cost-effective but not everything can be had in it, especially with the recent shorting of that class.
What I'd like honestly would be a generic 'Ark' ship that unfolds into 'That Room', by which I mean a generic Telletraan computer on one wall and a table/bed/repair bay thing in the center of an orange/yellow set. Very simple to make big enough to work with MOST things, while open-sided enough to not exclude stuff from the diorama even if they're too big.
Then maybe a Nemesis that can do the same thing, the purple computer on one side, a command throne in the middle.