Having bought prepared cars twice now, it is not a big deal at all. I always write a show bill of sale for $500 and then a real one for the actual amount. Actually at NHMS the BS judges saw the real bill of sale and I just told them that it was $500 for the car and $2500 for the safety equipment. If the car has been run before then you will not have a problem.
The way that I read rule 4.1.1 is that it is for instances where people are buying cars that were raced in other series and trying to pass a $4k car though tech saying it has a $3500 cage in it.
You can also write up the bill of sale for $500 for the car and then the balance for the belts, the seat, the fuel cell, and spares or whatever.
EDIT
+1 for making sure it has been raced recently. The rules do change year to year and a car that raced 4 years ago is no longer legal. I always bring a printed version of the tech sheet and to a tech inspection myself before I buy it.
Moot Point Racing - 1991 Volvo 240 - #496