All I can tell you - you're welcome to try.
You will quickly find out that it's not me "over-quoting" prices, but that your thinking is overly optimistic, unless you're okay with hacking together a barely functioning frankenstein, that doesn't resemble RS6 anymore.

There's no space to fit a proper FMIC, unless you get rid of the AC and relocate oil & PS coolers.
Exhaust manifolds may be able to flow 750chp, but finding proper turbos that will bolt there and flow enough - is a completely different thing. A pair of used TD05s might fit your budget, but you'll spend a lot of time making them fit.

"Various little bits" are the stuff that you don't count, but it quickly adds up. Gaskets, hoses, sensors, hardware, yadda yadda. Depending on how involved your build is going to be, it can vary a lot.
Bottom line of that message was: don't expect to "cheap out". Again, you can cut corners, but don't expect anything close to "LS community" prices with this car.

You didn't find any details, because there aren't many documented "big" builds. Usually the biggest thing people do - MT & hybrids. And that just barely touches 500whp mark. Try searching for "Milka build thread".