When you're talking about a $100 car stereo system, the "best brand" becomes a meaningless entity. At $100, they're all pretty similar. They don't start getting "better" until you're in the $250 - $300 neighborhood, and even there you're talking about minimal differences.

I wouldn't buy a $100 used car stereo. You really don't know what you're getting and have little recourse if and when the thing drops dead. Do it once, do it right or don't do it at all.

Start looking for the budget level iPod controlled head units. They allow touch screen navigation which is safer and for all intents and purposes, higher quality. If you can't go that route, get an "iPod compatible" budget model.