The 'Lemons Newcomers' section has several 'stickies', including "the best tips ...", with tons of information about building a car and getting started in Lemons. It also has a search function.
You can find previous discussions about:
Building/purchasing a car (SO many of this subject)
How much does it cost? (SO many of this subject)
[All of these ^ discussions with good information.]
If you _choose_ to build:
How To Not Fail Lemons Tech Inspection
Brakes: modifications, pads(shoes), and fluids
Recommended maintenance items to be replaced before going on track, like the giubo for BMWs.
A checklist of things to bring.
I haven't found much about the cut-off switch in the way of a preferred vendor, but there's plenty about how _not_ to wire a switch, and I think a couple of diagrams.
One note about "the best tips ..." regarding building a car if you choose to go that route: the priority list has "cage" before "seat".
Several experienced builders here suggest to _mount_ the seat _before_ building the cage.
Cages seem to take the most time to complete: finding a builder/supplier, getting it into their schedule, actually getting it back on time, and correct for Lemons ( How To Not Fail Lemons Tech Inspection ), etc. So, certainly planning for that part of a build needs to happen early.
The recently added requirement  for an onboard fire suppression system resulted in supply shortages due to increased demand, and therefore required the next longest lead time last year, but I haven't read complaints about that this year.
But I find the arguments for _mounting_ the seat _first_ to be compelling.