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I'm no flight engineer, but from what I've learned about Bernoulli's principles I'd guess that a basic car should be able to generate some lift with minor modification. Basically if you cause air to move faster on the upper side of your car than it does on the lower side of your car, you will create a lower pressure above than below. The result: lift. Since cars already are pretty flat on the bottom, and less so on top, the design isn't too far off from an airfoil, depending on the car.
So, if you were to put a full flat belly pan on the bottom of your car and completely eliminate air flow resistance, the air underneath you would remain relatively static. If you raise your bumper/air dam height just a little, you could actually compress air between the ground and your car (and change your angle of attack) which would increase the air pressure. The air flowing over the top of your car will have an increased distance to travel and will flow much faster than the air underneath your car, creating a low pressure area. If you streamline the top of your car to reduce turbulence, the flow will be smooth, fast and uninterrupted. If your car slopes down toward the back in a boattail fashion it should maintain this flow. If your car ends abruptly like most hatches, the turbulence behind the car will prolly create drag which may cancel out some of the lift.
A spoiler is usually put on race cars to interrupt the smooth flow of the air, reduce lift and add down force. Since your goal isn't to take sharp corners at 100+mph, you prolly won't need as much down force. However, if your car is light to begin with, expect to be pushed around by wind gusts when you are at speed. Also, sharp high speed maneuvering may be a little more difficult. If your car is decently heavy, it may just take some pressure off the wheels.
I remember reading an article in Car and Driver many years ago about the effectiveness of spoilers on common sports cars (not race cars). They found that most spoilers didn't make much of a difference at normal to excessive driving speeds with the exception of the Porsche 911, which became effective at around highway speed. If you look at the shape of that car when the spoiler is down, you can see how it might be able to generate some lift at speed.
Again, I'm no flight engineer, but the theory kinda makes sense, doesn't it?
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