Hi Mike - the PCR chassis is designed to take Slot.it parts. The Scalextric USA web store does, I believe, carry Slot.it cars and parts. In the UK, a store such as Pendle Slot Racing or Top Slots & Trains (my favourites) would be a good start. They would also probably advise on parts to use in the chassis. I'm sure that support would be available from any Slot.it dealer anywhere in the world.
Building a PCR chassis is an exciting and satisfying job and you learn a lot - I certainly have. It's worth getting the Slot.it parts catalogue (download here or get a hard copy from you local dealer) and familiarising yourself with the Slot.it concept and the range of parts available. It's not a cheap exercise!
The most important place to start is what type of motor you want to use and what orientation you'll be using. The motor, motor mount and gears will be specific to what you decide and are the most expensive components you'll buy (unless you're using a digital chip too). For the Z4, the choice is long or S-can, inline or anglewinder. For the Sierra, the PCR chassis just takes the inline mounts, either long can or S-can.
Last weekend, I had fun putting together a PCR chassis for a Ford Sierra RS500. This is what I used:
Scalextric C8546 Sierra PCR underpan
Slot.it CH114 Long-can inline DTM 0.5mm offset motor mount
Slot.it PA01-48 axle for rear axle
Slot.it PA01-45 axle for front axle
Avant Slot 20101 Hunter Long-can motor, 9T brass pinion & wires (from an Avant Slot car I've taken apart). The equivalent Slot.it parts would be: MN08CH Boxster2 motor; PI09 pinion; any silicone motor cable + eyelets.
Avant Slot 20605 Crown gear 28z (from the Avant Slot car) Equivalent Slot.it part = GI28BZ
Slot.it W16508215A Aluminium wheels 16.5 x 8.2mm (pack of 2) for rear wheels
Slot.it W15808225P Plastic wheels 15.8 x 8.2mm (pack of 4) for front wheels
Slot.it PT19 Z1 compound for front tyres
Slot.it 1207 P6 compound for rear tyres
Slot.it PA55 BBS wheel inserts (just use the silver wheel inserts and gold mesh)
Scaleauto SC1608 7mm guide (no good for using with ARC powerbases - Slot.it CH15 would be good alternative part)
Slot.it CH28 screw set to fix motor mount
Mitoos M171 copper braid
M2 6mm and 3mm grub screws to adjust front axle height (M2 allen key required).
I used the original body screws to attache the original body. I also used the DPR trap door cover, but left the wiring loom and light boards in the original chassis. I will eventually add a C8515 digital chip, but am waiting for some JST PH 2.0mm 4-pin female connectors to link the braids to the JST plug on the chip.
It's worth mentioning that PCR chassis for most of the current Scalextric models are available from the Slot.it 3D printshop on Shapeways: https://www.shapeways.com/shops/SlotIt
Have fun & good luck!