As said its a big job as it would require you to amplify all speakers/subwoofers and connecting a 3.5mm jack to RCA out on your amp. You would need a 3.5mm to 4xRCA cable seen it on ebay about £6-8, which would allow your nexus 7 to output the audio>amp?>speakers.
Apart from that usual wiring of speakers/sub to amp and powering the amp, not impossible but takes time and effort plus knowing how your gona mount the tablet e.g fiberglass custom fascia?
But i dont see it as a good idea in my opinion, sticking to RNSE or prob a cheaper alternative such as aftermarket headunit is better.
RNSE (G) latest edition is about £350-400 for good mint condition ones, ? If you check as said above MightyCarMods on YouTube they did the iPad fit its the same process see the video and decide what you want to go for.
And yh it would be great to have a tablet fitted but again needs time/creativity and a few mods, but am looking forward to it to see what you would do.
If you search eBay specially the auction section you could bid for some mint ones if not then have to go for the forum members here. My bother bought one for £390 free P&P last week, so shouldn't be a problem finding one.
Personally I would say that is very cheap. A little too cheap if you ask me. eBay isn't the most reliable of places to buy those units. Can never truly guarantee if it came from a legit source or not. You just need to read the sticky about stolen RNS units.
I also have to agree with Ash about the pioneer units they are pretty sweet for the money.
Im going to be attempting a mod sometime this unit to incorporate my iPad2 into the car whilst keeping the RNS present. I also have a guy building a Raspberry PI into the car that will be controlled using original car controls. Will allow for XBMC, DashCommand (OBD information on screen), SatNav etc etc.