You are right! I double checked and the specs says that ECC mode is disabled only with some CPUs (for sure the ones that doesn't have support for it).
In my country G6600 and G4560 are pretty cheap sometimes (both between 50 and 60€) so your build seems pretty cheap considering this is all new hardware.
The only thing that holds me back is the limited number of cores (I really would like to have a couple VMs) and the pretty high cost of ram (100€ for 8GB on a single stick and 110€ for 2x4GB).
Anyway I'm really considering this build. Thanks a lot
@TomUK for giving me peace of mind about this MB and about the 8GB for FreeNas, they were two unsolved concerns for me!
For now I think is still better for me to get business grade hardware (used) to get the opportunity to play with multiple network ports (with dedicated network management if possible) and to have a modest number of cores to play with VMs. Also I don't have a PSU and even the case that I have is for sure not the best for ventilation on a 24/7 machine so buying a bundle with a good case (or possibly rack) is definitely a bonus.