Last time I spoke to Marlin, Terry told me they were moving to a model where they concentrated on the core kit and supplied a list of recommended components for builders to source separately. She said they could still supply a complete set of parts for those who really wanted it but that it would work out more expensive (obviously as they will put a markup on things).
On that basis, I'd keep the parts you get from Marlin down to the must have items to fit the kit such as the windscreen (if you want one that is). I can't think of anything else that won't then be down to your personal taste or the particular requirements of the engine installation. Things like the instruments, wiring looms, swirl pot, fuel pumps, senders, lights, exhaust etc really need to be specified to line up with your particular car rather than a generic kit.