With 4x 2TB SSDs we’re already at 480€

Not sure that would fit in the budget

Xbian uses upstart

Go with Mint, it’s my daily drive on both my laptop and HTPC. If you choose the regular edition Ubuntu based you have also hardware enablement (hwe) kernels which could be useful on newer hardware.

Rock64 had 4GB max

Pretty standard with Mexican manufacturing plants

You could have just done a little search https://orgmode.org/