What to get for android compatibility, bluetooth & can hook up 4 cameras

Got a '12 avy and no idea what head unit to get.  Nav is not critical as I always have my phone but wouldn't hurt and want to be able to connect to my Android phones via BT and hook up 4 cameras.  I plan to have 1 each facing front/rear/both sides.  I'm not an audiophile, the stock unit with bose sub sounds fine to me so as long as it's equal I'm good.  I don't have a lot to spend so is around $500 or so possible?

You have a lot of options; Crutchfield would be a good place to start and you might be able to find something on that budget.  That being said, not all head units will be "plug and play" nor will you be able to retain some items such as Onstar, steering wheel controls, and/or XM. 

This unit looks like it might give you what you are looking for:

http://www.amazon.com/Replacement-Touchscreen-Navigation-Chevrolet-Installation/dp/B007VGDHL0/ref=sr_1_1?ie=UTF8&qid=1400843669&sr=8-1&keywords=oem+nav+chevy+avalanche

I do not have any experience with this particular unit but it does have pretty good reviews and it looks just as good, if not better, than oem.

Thanks Galaxy.  It's out of stock now but I can probly find it somewhere.  Talked to Crutchfield and they had no suggestion for 4 video inputs and to use some sort of switch.  No idea what to look for on that front but for everything else they suggested the following:
http://www.crutchfield.com/p_105KWV30B/JVC-KW-V30BT.html
http://www.crutchfield.com/p_794ADSMRR/iDatalink-Maestro-ADS-MRR-Factory-Integration-Adapter-also-requires-iDatalink-vehicle-specific-harness-sold-separately.html
http://www.crutchfield.com/p_794HRNGMS5/iDatalink-Connec-ADS-HRN-SR-GMS05-Factory-Integration-Adapter-also-requires-iDatalinks-ADS-MRR-module-sold-separately.html
http://www.crutchfield.com/p_220SXV200/SiriusXM-SXV200V1-Tuner.html

I just installed an Android-based headunit in my car and it is AWESOME. Have it hooked up with a video switcher for my cameras (rear camera and RV camera). Since it runs a base android - all my phone needs to do is share a hotspot to it and boom - can access anything on my headunit that I could want from the Play store. Great device!

http://www.ebay.com/itm/Android-4-1-2-Din-Car-DVD-PC-Head-Unit-Stereo-Radio-GPS-WiFi-3G-CAPACITIVE-NEW-/321256491963?pt=US_Video_In_Dash_Units_w_GPS&hash=item4acc610bbb