So, little update...
Decided to finally bite the bullet. Sold my S4 Biturbo motor - the new owner drove down from London, armed with a box van, a pallet and an extremely large sidekick, paid for, and took the engine away.
I've now purchased a 4.2L V8, from an early S8, (non-DBW) so 340PS to play with, before things hot up later with a turbo/s set-up.
It's complete with loom, ECU, all ancillaries, instrument cluster, airbox / MAF and original ignition key.
It's of course, an auto, so will be ordering a V8 flywheel from Audi. I WAS looking to go aftermarket ECU, for ease of installation, but decided against it, for a number of reasons:
1) I'll lose my on-board computer function
2) Not sure how the variable intake manifold would work
3) I couldn't use VAG-COM with my OEM OBD connector, not for the engine anyway
I spent Saturday going over the S8 and my B5 schematics, and I can actually integrate both, and retain the OEM functions. I can also keep my OEM instrument cluster too, as cars prior to '99 didn't have the immobilliser incorporated in the dash.
I might use the S8 one however, as the V6 tacho will be far from accurate with a V8 motor, and the on-board computer will DEFINITELY be well out, in terms of fuel computing.
Engine and all the hardware's being delivered on Saturday, and I'll probably get the intake plenum, rocker boxes and timing belt covers done in wrinkle black / grey.
Updates via the usual pictures and text.
Picture of the S8 motor still in the car, as at last week...