But is it necessary to flash another firmware on the SAS HBA to use it with OMV?
No. Lots of people flash the IT mode firmware because they don't want to use raid but this isn't required.
r are there any documents about SAS and OMV that I can use to read through and get my open questions answered?
There is really no difference between SAS and SATA from the OMV point of view.
Do you recommend to get these two disks and build my first NAS based on SAS HDDs or do you say that I should just buy the usual WD RED HDDs and build it as usual.
Most SAS drives are supposed to be better quality (higher MTBF) than SATA drives. So, it they are free, it is a very good choice.