2009 Stereo Upgrade

Alright, 2009 Bose stereo but no touchscreen, the plain style head unit.

Can I just plop an OEM touchscreen unit in there? I don't really care about navigation. I don't seem to see many for sale and when I do they are like $700, not worth it to me.

Should I just go with an aftermarket head unit and the right adapters to keep the sub and steering wheel controls? How much is that going to run me?

The one thing I want to be able to do is put a ton of MP3's on a USB stick and connect it to the head unit. Bluetooth for the phone would be nice but I'm going to assume any aftermarket head unit will have that. Do the OEM touchscreen units have a USB port? Looks like some do and some don't from pictures online.