Time Capsule and NAS DIY with Raspberry PI

Hello. First to say sorry if this is wrong forum. I need advice for setting up my network. I have one iMac with 320gb hdd and one MacBook Pro with 250gb ssd. The hard drive on iMac by the time became little and the other data (documents, etc) I used to have it on old laptop with hard drive, now do not want it on SSD.
Part one.
Make NAS from openmediavault linux.
1. To move the iTunes library to NAS, relocate the iTunes folder from both computers to single library on NAS.
2. To move the Photos app library to NAS and launch it from there. Since Photos app is not used often, it would be ran on one computer at the time only.
3. Move other data to NAS and mount that network share at the boot.

Second part.
Create one network share to use it for time machine backups from both computers.

Idea is to use single board computer like Raspberry Pi and hard drive(s) for file sharing, to upgrade my wired network to gigabit and upgrade wifi to N or AC (currently 802.11G).

I already have few hardware, but I want to hear your advice first. I have Banana Pi (USB 2.0, SATA port, Gigabit LAN) and I have the USB 3.0 external HDD enclosure for two hard drives (RAID, JBOD, only one).

Is it better to use SATA with only one hard drive? The RAID on USB 2.0 standard? or combination of both. for example one for time machine backup and other for storage.

Thanks in advance!