Truthfully, I'd look for some later cars with full interiors and swap shells. I suspect that gathering all that stuff and building them will be a lot more work and probably more money. I regularly see a set of of Premier Madison cars in the $200 range. The overhead lighting is easy, convert to LED lighting for around $12/car using my LED Lighting Regulator boards and strip LED lighting.
As a bonus, you could probably drop the shells on your old chassis and sell them to recover some of the money.
