Just go for a browse on Craigslist or kijiji (if you're Canadian). You'll be able to find a half decent head unit for around $50. That will save you some money for your wiring adapters that you will need to buy/make. You don't need to go balls out when buying your head unit, because by the sounds of it the rest of your stereo isn't to high quality either. You don't need top end, by try to stick with some of the larger brands.