The best thing you can do is look at HCL's ( hardware compatibility lists ). Each distro has them, since users will report what works, and what does not work on a distro.
If you are trying to sell this, pick one or two main line distros to offer. May I suggest you look at Mandriva. Their hardware support out of the box is as good as I've seen. There are others that are close, not better.
Each area of the system has its problem manufactures. Video cards and audio cards are good examples. Wifi cards are a nightmare. Most issues show up after an install. Ie, the xserver won't start.
Do some reading on this forum, there are lots of examples of the types of problems people have. Hardware support for linux is outstanding, considering the tens of thousands of cards and chip sets available. Knowing what driver to load is a huge problem for a knob.
Have a look through the Tutorial section on this board. Lots of good stuff there to help with problems.
Any new user to linux has a learning curve. So, you will have some learning to do. Take the time, it is worth the effort.