As I mentioned the other questions are off-topic, I'll stick with answering the fourth question you posted.
The RPi can be used as a network attached RAID storage device, but there are a few things to consider
Power:
At this point, the RPi 2 and 3 are usually able to power a single external hard drive without issue. You need to pick a lower powered drive, and a decent power supply for the RPi, but it can be done. That being said, the RPi will probably be unstable with 2 external hard drives attached, I doubt more than that would work at all.
You're going to need an externally powered USB hub for this to work.
Throughput:
The RPi's USB bandwidth is shared between all four USB ports and the ethernet controller. If you're trying to write to multiple hard drives from across the network, this is a huge performance hit. This can be alleviated somewhat by using an RPi 3 and connecting via Wifi, but I wouldn't consider that a preferred option for a NAS device.
In contrast, an actual NAS device will deal with SATA/SAS connections, which will give you much better performance.
Magic:
Okay, so nothing is actually magic, but there's a lot of things built into a dedicated NAS box that you're just not going to get in an RPi. Things like hot swapping, and the user interface are a big plus provided by the commercial solutions. You can get some free software that has similar features, but you're just not going to get the hardware level control from it like you would expect in a full sized NAS.
Hardware vs Software:
The processor in an RPi 3 is more powerful than what you'd find in the more inexpensive consumer grade NAS devices. That being said, a lot of that horsepower will be dedicated to the fact that it's doing a lot of work through USB (network and attached storage). Additionally, your device redundancy will be handled through software. There's no way around that.
A proper NAS device will have the redundancy writing system built into the actual hardware of the device. They don't need as much horsepower simply because it has better purpose dedicated hardware.
Benefits of the RPi:
While it's not the best plan for a NAS, if you want the device to do multiple things, there's certainly room for the RPi to shine. Unlike a NAS, the RPi is a full linux computer, with a wide range of I/O connectivity. It could simultaneously host a webpage, or other project of yours that doesn't require the RPi to continually move.
I've always thought it would be cool to put a temperature/smoke sensors on an RPi and have it do an emergency off site backup if it detected a fire. Plus, learning is fun!
