Thanks everybody. I'm not in a hurry so I'll wait.
So I've installed Recalbox and it is really as advertised : just copy the files on a micro SD card and it works. I've added a spare BT dongle to make shure that my 8BitDo controlers would work properly. The only difficult thing was to format a 128Gb card in FAT32.
I'm now in the ROM and BIOS stuff? I grabbed full rom sets (reference fullset for MAME178, FBA reference full set, and the No-Intro ROMs, as well GoodRom fullsets for several missing system) but ouch... I end with thousands of games, most beeing forks of others. I'll need to figure out a way to (automativally) reduce... I'm investigating ClrmamePro but I cannot make it do what I want. I through that my ROMsets ware standardized, but ClrmamePro seems to struggle pairing DAT files with my zip files. I thing that I'm actually starting the diffucult part, afetr the flawless installation of the Recalbox.
About Kodi and Plex : the desired extension installs and a sync with my Plex server is currently occuring. So I'm optimistic.