New OMV user, new build

I have never had a NAS and never used any kind of RAID. I am still going trough various how-to's, so I apologize if I ask something that may have been discussed before. I have about 19tb data at the moment, without any backup. As backup is not an option, I would like to make my data more secure using NAS storage system. At some moment I have been thinking about a pricey option of a server, large quantities of ECC ram, zfs file system with multiple zpools, ... ; but there are some characteristics of ZFS that I did not like when I learned about them. Then I compared the usual suspects (unRaid, Freenas, Nas4Free, Amahi, OpenFiler, ... , bare Linux OS) and OMV was the one that I liked most.

I have some hardware that holds the data now in form of NTFS drives in Win7, that I intend to use for the build: MB Gigabyte B85M D2V, 4GB RAM, Intel G3220. Other elements I consider are Antec 902 V3 case with Silverstone CFP-52B bay devices for 12 drives, a RAID card and an 80Plus PSU. I have been thinking about redundancy and RAID 5/6 does not do it for me. I have now the situation that when I lose a drive, I lose only the data from that drive and I want to keep that commodity so RAID 1 (or perhaps RAID 10) is the only option that I can consider. I have some RAID oriented questions, but I'll ask them in the appropriate section. The idea is to build the rig step by step, buy two drives create a mirror, transfer data from stand alone disk to the mirror, recycle old disk, and all the way around until I finish it all. At the end there should be about 48-56TB on 10-14 WD Red drives, exact number will depend on price difference between 4tb and 6tb drives in three to six moths. I have created a virtual machine of OMV with a few virtual drives and added them as a mirrored array, installed a few plugins, created some share folders, an user, created shares in windows and everything looks nice at the moment. I want to make a real demo machine next and try with a few real devices. I have some 320gb mobile drives that could help out for a few days.

A few quick questions, how much resource hungry is OMV with soft RAID with so many disk (processor power, memory usage; should I add 4gb more?)? What ball figure transfer speeds should I expect when using the OMV as mapped network drive in a Windows 7 PC with gigabit network?

Another 4gb RAM is not a problem, i'll add it.

I will not use the same drives where the data now resides. My plan is to buy drives in sets of two, format them, make partitions, add them as mapped drive to windows, copy data from a NTFS partition and a few day latter if everything works fine, wipe the old drives clean and sell them. Then a months latter buy another two drives and do the same routine until all data gets to OMV.