basically I'm after a new head unit that can play my iPod, has at least a 6'' screen and if possible has the ability for android phone connectivity and sat nav but not essential. and obviously as little cost as possible.
If you're running the Bose setup it's not just a simple plug/play job. There's all sorts of adapters and weird shizzle you need to install too - just a heads up.
Search for app radios. Ie pioneer da100 or da110
I went through all the rigmarole of putting an aftermarket HU in my S3 a few months ago; in the end I decided it wasn't worth it and went back to the stock HU (Chorus II, with a tape deck!) and a Connects2 box behind the dash to handle iPod/Aux inputs.
For the aftermarket HU, you'll want to make sure you get one with dual (front+rear) pre-outs on the back, that way you can use the Autoleads PC9-408 to connect to the Bose system, far better than using that high-to-low converter lead. There are a few problems however: You'll need to run an ignition live from the fusebox to the HU because the stereo won't know to turn off when you take the key out of the ignition. Also you'll probably get loud pops from the speakers when turning the HU on and off, although that can be fixed by fitting ground-loop isolators between the pre-out and the wiring harness.
All in all, it's far easier to use the stock head unit (no popping, turns off and on with the ignition) and use a box in the dash to handle aux/iPod/bluetooth