The black Cadillac Superior Crown Royal Landau 3-way is a 1/18 diecast made by Sunset Coaches. It's the wifes, so it'd be harder than converting one of my diecast hearses.
It would mostly come down to finding a rear-end or mating gearmotors to the wheels, then either mating a servo to the wimpy steering components and upgrade as required or source/make knuckles and a steering rack. Decide how much interior detail to lose and where so you can hide rc parts... worry about the beautiful fragile detail parts anytime it moves.
The white one is already toy RC, so it would just need a servo, esc, rx, battery.
Then i'd move on to: shave the ecto junk, probably sectioning the body several places to widen and stretch it, section the chassis, repaint, new glass for bigger windows, building a new grill...
Not quite the 7 foot wide, 21-23 foot long professional car i had in mind when i ordered it. I had been thinking black / metallic grey with bright aluminum chrome, regular lights, blue or purple underbody lights. Maybe even throw in a couple extra servos and go lowrider. But it's out of scale.
I like hearses.