I agree with Morris - I bought only the things I really needed from Marlin (bodywork, chassis, windscreen and the front upright modification that they needed to do for an E30 car), and sourced everything else from the following places:
Lights -
Stafford Vehicle Components (SVC)
Assorted Parts -
Car Builder Solutions (CBS)
Brightwork & Other Parts
ETB Instruments
Seats -
Interiors Seating
Electrical Cable and Connectors
Vehicle Wiring
Plus keeping an eye out on eBay.