So, interesting - EHW goes with the PD115 engine in the B5, for one year only, mainland Europe only. I'll get my coat
That has a 4.111 rear diff so from the D2 range you would need an EUU which is the S8 diff, although confusingly the data shows the A8L may also have that so you might not even need a diff.
Also interesting, with a 4.111 diff, that 01E might not be too high geared so may well work nicely. The only downside of it is that it is the small sump version without an external oil cooler. All of the high torque applications of the 01E have a larger sump under the diff and a diff-driven oil pump to an external cooler.
The only A8 4.2 I know of with a manual swap used the S8 bits in order to use the S8 file, but it is entirely possible to make a manual map if one is suffciently skilled. It just tends to be rare and expensive, but if you can do it then go for it
The TTV flywheel was £370 last time I bought one. It may be a little more by now. The RS4 clutch is under £200 I think. The S8 original manual flywheel is single mass so the heavy TTV works fine. There's no need for a dual mass.