It is not correct to say that cars that will require an IVA are built from kits where everything is supplied new.
Some are built from comprehensive kits (like Caterhams), many are not.
It is quite possible to scratch build a car and successfully pass IVA.
Many builders build their car's chassis themselves and fit used parts from breakers yards.
See
http://www.locostbuilders.co.uk/
For example:
http://www.locostbuilders.co.uk/view...php?tid=194553
It is nothing to do with me, but I follow the build thread because I admire the amazing standard of work.