It bolts streight up to the V6 (or 1.8t) gearbox.
You need a flywheel, ttv-racing do them, or you can buy an adaptor for the flexplate from 034 motorsport. these then allow you to use a standard S4 or RS4 clutch.
The V6 subframe is better, the 1.8T subframe has a "deeper" section which the engine hits. You need to slightly modify the sump casting by grinding off some ribs, but otherwise the engine fits. You will need to fabricate some engine mountings, but this isnt too difficult if you can weld.
As for the exhaust, Siena found that his V6 system bolted streight up to the V8, although this will depend on which V6 you start with (his was a 2.6) The V6 system will undoubtedly be too small though, the A8/S8 runs a twin 2.5" system right back. You'll find it will be cheaper to get a custom system fabricated, than buying a milltek etc.
The wiring is actually very simple tbh, i'm fitting an ABZ to a Landrover, and have stripped the OE harness down to remove the Autobox loom, i'm left with about 20wires that need interfaced with the car, but they're all very streight forward things. Just make sure you get the Immobox and a key with the engine, that way you can be sure to have a working immobiliser for testing.