Just browse this forum for ideas. Basically, anything will do. Try to stick with a "mainstream" manufacturer like Pioneer/Premier, Alpine, Sony, Panasonic, JVC, etc.
There are plenty of new options for around $100, and you could always check eBay or Craigslist.
Spend a few bucks and get what I did . Had the JVC for a while now and its a great deck ,not one single issue . This deck does a lot of cool things ! Go double din and forget filling in the space from a single din , looks like shit .
ok in my old car i had a CRAP head unit and my system sounded like crap, all i did was go to best buy and spend 80$ on a decent head unit and was amazed at the difference. I told a friend and she did the same and was also amazed. you dont need to spend hundreds, around a 100$ would be fine and it comes with most the features like FM/AM, CD, AUX, USB and all the other stuff.
If you live in a major urban area and listen to a lot of AM radio, you may want to consider getting a HD radio unit. It makes AM sound like FM, if your station supports it. HD FM stations sound close to CD quality.
I think this is a great price on a good head unit with some nice features like 3.5mm jack and usb input. The ipod adapter is a little pricey but that's expected with anything apple related. This unit will have a clean, stock look with lighting that matches OEM. I recommended it to a friend and it looks great in his Camry.