Suggestions for an affordable NAS for backups and Plex (media center)

Hello everyone,
i've got this in idea in mind and i need your suggestions: i would like to buy a NAS that is reasonably cheap but totally reliable and that can be my Plex media center too.

I want to put all my important data there and i want to be 100% sure that i won't lose them. So far, i'm still making DVD of the stuff i want to keep for a long time, and using an external drive for "light" stuff, like music, and a WD TV Live with an external drive for "heavy" stuff, like movies and TV series.
I would like to use Plex as media server and media client so i'm oriented towards a QNAP, that has the integrated Plex app and is cheaper than a Synology.
I was thinking about a little Raspberry Pi with RasPlex (http://www.rasplex.com/) linux, connected with HDMI to the TV and with wireless or ethernet (i can't decide) to the LAN.

I was considering the Qnap TS-212P (http://www.amazon.com/TS-212P-Personal-Network-Attached-Storage/dp/B00G2HZNXO/ref=sr_1_1?ie=UTF8&qid=1402523008&sr=8-1 / http://www.qnap.com/useng/index.php?sn=862&c=355&sc=688&t=696&n=19604)
It's cheap, 173 dollars, it has 2 bays, so i can do a RAID 1, but i'm not sure it could do some good transcoding when needed because of CPU (Marvell 6282 1.6GHz) and RAM CPU (512MB DDR3)

I've got a Western Digital Elements 2 TB USB 2.0 Desktop external disk (http://www.amazon.com/Elements-Desktop-External-Hard-Drive/dp/B002QEBMCI/ref=sr_1_2?ie=UTF8&qid=1402523455&sr=8-2) and i could open it and use it as one of the disks for the NAS.
I also have got an external USB Western Digital My Book 2 TB (http://www.amazon.com/Book-USB-Hard-Drive-Backup/dp/B00E3RH5W2/ref=sr_1_1?ie=UTF8&qid=1402525023&sr=8-1): is it a 3.5' drive? I could open it, so i'll have 2 free disks.

Then a 40 dollars Raspberry Pi (http://www.amazon.com/RASPBERRY-MODEL-756-8308-Raspberry-Pi/dp/B009SQQF9C/ref=sr_1_1?ie=UTF8&qid=1402523619&sr=8-1) would be the only thing left.
Total cost: 173 + 40 = almost 210 dollars, pretty affordable.

Now, the point. I've got some doubts:
01) Is it a RAID 1 a good choice for reliable backups? It's simply a mirror, so if both disks don't fail at the same time i should be pretty safe, shouldn't i?
02) What other kind of RAID or storage could i do with a 2 bays NAS?
03) Considering that i should already have 2 free 2 Tb disks, i could buy another Western Digital Green 2 TB SATA3 (http://www.amazon.com/WD-Green-Desktop-Hard-Drive/dp/B008YAHW6I/ref=sr_1_2?ie=UTF8&qid=1402523554&sr=8-2) for 82 dollars and then a 3 bays NAS. Could i have more space like that? Maybe doing a RAID5 using 2 disks for data and 1 for bit redundancy, so that i have 4 Gb of space?
04) Is the CPU of the Qnap TS-212P good enough as a Plex server?
05) Is a Raspberry Pi good enough as a Plex client? Could it work in wireless? I can download up to 5 Mb/sec (40 Mbit of bandwidth) with my wireless N router.

Sorry for all those questions. I know it's a long post but it's an intriguing project and trying to keep it cheap isn't so easy. Besides, i'm a total newbie in this stuff.

Thank you in advance.

Thank you for your suggestions!

Right now, i'm considering the idea of buying a NAS and using it as a storage location for a second copy of my important data. I would like to backup my important stuff on the NAS but keeping it on the PC anyway.
And i'd want to keep the heavy uninportant stuff, like film and tv shows, only on the NAS.
It's always better than the two separated external hard drives i'm using right now and i could create the Plex media server, for music and videos.

Isn't that a decent idea?

A direct question: if i'd want, let's say, an amount of 4 Gb total of storage, but i'd also want to be sure that if an hard drive fails i don't lose any of my data, what kind of RAID i should choose? And with how many disks? How large?

So i need a 4 bays NAS, because i can't find any 3 bays ones anyway, am i right?

It's not gonna be a so "affordable" project with a 4 bay NAS but i will deal with it

I really must study well how the different type of NAS work: in RAID 5, with 3 x 2 Tb drives, if i add another 2 Tb disk, i will have 6 Tb, without losing any data if one breaks?