To answer questions about parts availability, I'll add these suppliers ONLY because I'm familia with their products through purchases I've made for the VRAA proxy builds. I know most of the retail advertisers and sponsors sell scratch built parts, including from the sources below.
D'Art has the 13" and 15" narrow wheels and tires and chassis parts that would fit the specs., as well as inserts, driver figures, etc.
http://www.darthobbies.com/BWA also carries wheels, tire and inserts and chassis parts:
http://www.bwaslotcars.com/From experience the MRRC plastic inline adjustable chassis is an excellent choice for fitting under these tiny cars, and of course it came/comes under many different RTR cars, so they are plentiful.
Penelope Pitlane chassis look nice, I've never built one, but they are currently available.
I.ve built a couple of PC-7 plastic chassis, they work well and awill fit under even the smallest bodies.
Just suggestions!