What Tech says should be engraved onto the forehead of every rodder who ever tries to start a build. When we start a build, we start with a spreadsheet. At the top of the spreadsheet is the theme of the car. IT may be "build a car period correct for 19xx" or some similar goal. Then, under headings such as "BODY", "ENGINE", "DRIVETRAIN", "INTERIOR", and "ELECTRICAL", I list every component that I can imagine the car to have. Thus, I am making a list of components. Then, next to each component, I list what it is and a source so a line may say "steering wheel" "1939 banjo" "have in stock"
This spreadsheet is an evolving document that changes over time and as you find parts. My spreadsheet was about 10 pages long for my 39 coupe and was shorter for the 32 roadster. As we continued the build, we referred to the plan....was the part consistent with the theme? This helped prevent a fat GM steering column from getting into a retro build. IT also prevented us from straying from the theme as time passed. The document was started before the first part was acquired....just like Tech said. Cams, wheels and tires, converters, etc. are the LAST items.....even after paint and interior. Too many novices get excited and order wheels and tires first and then find them outdated or find cooler ones when the car nears completion. Start with a plan is the best advice ever for a car build.