Well contrary to popular belief the chorus II+ doesn't actually have outputs for the rear amp either, i had to replace the chorus with a concert to use the newly installed rear sub.
The amp is built into the SUB, so unless your putting in a 3 way amp/SUB setup then I would stick with Audi system, you need to buy the sub for the shape of your car, normal or sportback, two speakers and all fixings to mount them into the doors etc.
The car I worked on was a normal car, I bought all the fittings and connectors/repair wires from a dealer and then made my own wiring loom up using the repair wires and my own additional cables. Yes you will need 2.5 cable for the power, I got that from the fuse box at the front right.
So cabling wise you need
Screened cable 4 way to reach the radio in the front for L&R and remote
2 wires for power, +12V to fuse box front of car right and one to ground local to SUB.
2 wires for CAN hi & lo cables to reach the radio
4 wires for speakers in rear L & R