OK, so I found this in a recycle bin....But alas, nothing to loose, I decided to buy a working logic board, because some mold was present on the old one, and to just give her a go and learn what I can.
Anyway, the new board arrived, I installed it without any obvious trouble as I did successfully before with a 2008MBP, thermal paste and little connectors and all.
Since the new battery did not arrive yet, so tried booting it just with the power adapter and a known good HDD.
Absolutely nothing happened....
The only questionable thing during the new board install was the keyboard tape connection, because I simply could not make out if it was properly seated.
What would be my next step in this puzzle?