You can understand "off-site" in a much simpler way: buy an external USB2 hard drive that you can use for secondary back-ups. Once that back-up is done (once a week/month, depending on your needs), just dump that drive at a friend's house. Pretty cheap off-site back-up!
Backing-up is crucial, but don't go obsessive compulsive!!! Chances are you're never going to need it. So you can do daily back-ups on a internal or external drive, and have a weekly/monthly back-up that you physically get off-site. Simple as that.
Don't confuse fire resistant with fireproof. If your house / apartment building goes up in flame, that drive will be dead. It's rated at 30 minutes. But I'm pretty sure that the temperature will kill your data / warp your drive in any case.
I mentioned those and I chose them for the very simple reason that having a RAID will slightly increase your chances of something bad happening. Again, don't over-worry about that "slightly". After reading, I discovered that the RE drives would off-set that increased chance of failure. But it' up to you.
More ram is always a good investment. Always. Then again, paying the huge premium for big chips right now may or may not be justified. Try to ask around, find people that do pretty much the same thing you do and ask how much ram they have.
IMO, you're better off with more RAM than either a 2 SSD RAID0 boot (compared with a single SSD), or a 4 disk RAID0 (compared to a 3 RAID0).
Loa