A couple of little jobs to do then
11/10 for taking the engine out and leaving the downpipes in place
That's not an easy way to do it. You do seem to have a pretty good score on WITW bingo though.
I've only once heard of a 5HP mixing diff oil and ATF. There are a bunch of seals between the two areas and failures appear to be quite rare but not impossible.
Those spark plugs look like they belong in a flathead Ford V8, not an Audi V8! I don't think I've seen Champion plugs in the last decade tbh. Bosch or NGK are OE in most VAG stuff I encounter so I stick with them. Bosch FGR6KQE are cheap enough.
Oil cooler failures are pretty rare. I'd leave it alone tbh and just change the plastic pipe for an aluminium one.
Are you sure its the rear main leaking and not just oil dripping down from the valley? Its 50/50 for the rear main to leak, but 100/0 for the valley leaking and dripping down the back. If it is the rear main, the Elring seal for the 2.5TDI is a straight fit, rather than replacing the whole rear cover (for £300) which Audi want you to do! Elring Dirko sealant is the special sauce for the sump and cam covers btw. Same stuff as OE but 1/4 the price, and in a better package so it lasts longer.
Its worth noting that UK supplies of stuff for D2s is almost dried up, with those remaining suppliers who have stock gouging on price, and those who used to have good prices now taking weeks to get stuff from Germany. I'm having to order from Autodoc a month in advance now because they're the only ones with stock and sensible prices, alas at the expense of the usual suspects in the UK who can no longer compete. I hate Autodoc but they've become the only viable option.