The regressive nature of a Fixed Rate National Sales Tax can be addressed by a Graduated National Sales Tax. Exempt, for example, the first $10,000 of an automobile purchase. After the first $10,000 the tax kicks in at 10% for the next $10,000 of the cost, 20% for the next $10,000, 30% for the next $10,000 and so on.
I can't get excited about a flat tax as it'll keep the IRS in place at its current size and could be easily increased at a later date.
I completely agree, if a National Sales Tax of any sort were adopted, that it would have to be contingent on a permanent end to any form of income tax.