Need for speed underground allows you to customise and race cars, but I can't think of any older games (except maybe Death Rally, where there's an upgrades shop after each race).
Yikes, I remember an even older CGA 8086 program called "Car Builder v2.10" with a copyright date of 08/02/1990. It had a 2D, side-profile "body" design component and selections for engine, chassis, and suspension (at least) plus wind tunnel and road testing.