Welcome!
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
Cages/suppliers
Seats
Fire suppression
Harnesses
Helmets
Driver's clothing
Driver cooling
Brakes: modifications, pads(shoes), and fluids
Tires
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 [2019] 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.