new car prices.

Your reasoning about depreciation is spot on. It hurts to by a new car and have it worth less the moment you drive it off the lot.
A better idea might be to find a slightly used car that has already substantially depreciated.
There are couple of flies in the ointment in building a custom car to defeat depreciation and then driving it in traffic.
One thing to consider is the trouble with obtaining adequate agreed value collision insurance.
And then if you are hit by an idiot driver, you have to deal with getting the insurance company to pay for proper repairs.
Ask me how I know.