Congrats! You have a lot of searching to do! Pro tip: use google to search this site like so: "site:rs6.com oil cooler pipe"

I'm interested to hear what is causing oil in the coolant. Hopefully it's just the stupid plastic pipe - the replacement you want is from James Bufkin.

The best fix for the buttons I've seen is sourcing a pre-facelift (98-01) climate control unit with the hard plastic buttons and swapping just the buttons over. It matches well enough and there's no more soft-touch junk to wear off.

For parts, I'd recommend sticking with OEM where possible. There are a ton of sites to purchase from but I usually use: Europaparts.com, FCPEuro.com, Amazon, and some of the dealers that sell online.