I'm not familiar with any cars that can be unlocked with IR LEDs, RF and keys are the only two methods I'm familiar with. You could possibly make an RF device to accomplish that, but it would require a ton of research to determine just what frequencies different manufacturers use, and how the code they contain is transmitted, and how to make a counter to go through all the possible codes. And that alone might not be enough, if I were a car manufacturer, I would include a security feature that would detect such a brute force hack, and lock down for a bit. All in all, it would be incredibly difficult. Car manufacturers didn't make these locks so that people could make a simple device to hack them.
Just to comment on your business idea, if you were to create such a device, that alone would not be enough for a business. You would probably have to learn the whole locksmith trade to earn a living. Or you could sell such a device to locksmiths.
You can take a look at the projects page for the garage door hacker, that seems related to what you are describing. But I don't think the device you want is very realistic.