I like what you've done so far & your approach.
As to the cladding (for the false frontage) I suggest:-
Pick as many cheap kits/broken or incomplete. Use those bits.
Look at cheap, again broken or incomplete, toys. Use bits from those.
(Charity shops/car boot sales/e-bay/Internet recycle sites are all worth a look)
With a box full of bits, & a hacksaw to break up the larger shapes, buy some
craft scissors (the ones that cut out different edging). Sit down one evening
& while you watch a film/interesting programme cut up a cerial packet or two.
This will provide a tub of thin card with interesting edges, so that when you
start gluing together your false fronts you can fill in the gaps with the thin card.
Even better if they overlap, as that adds to the 3D effect when you dry brush later.
Looking forward to seeing what you come up with.