Recently, I was considering what car would be the cheapest possible transportation, over a lifetime, and came up with the Nissan Tsuru: the early '90s-era Sentra still sold new in Mexico. 2000#; $9,500; EFI but without expense and complexity-increasing stuff like throttle-by-wire, 27 separate airbags, etc.
The downside is that the NissanB13 Sentra isn’t sold new in the US, and you cant import non-conforming cars newer than 25 years old into the US. Past precedent with the Mexibeetles says that as long as the firewall and floorpan remain from an older version driven down, you’ve just “repaired an existing vehicle” and you are legal to operate your '71 Superbeetle home.
So, can you replace the firewall at a later date, or do you need a new VIN? Same with a rusty floor: is it a “irreplaceable” part or not? (Like the story of the farmer who had “the same axe” for 40 years: replaced the handle twice, and the head 3x, but still had the same axe!)