I would put more weight to say 4x 250 GB drives, or something to that effect, rather than TB drives in RAID. Like robw said, it’s a lot to loose when drives go, and they will, just a matter of time. I also generates a bit of force on your organization of material, which in my case is a good thing. Finding needles in haystacks was never my thing.
This would also be much cheaper and if there are money constraints, the price difference would easily afford experimentation with for example drive swapping, for a speedier workflow between projects.
All storage medium does come with limits. The only way to find out if this would prohibit your way of doing things, is to try it or try to calculate it. I am doing fine with working on projects using SSD’s. This may not be true for everyone.
The Samsung EVO drives I mentioned have a nice “lifespan” (look for that title on the link).
I look at it this way, a traditional hard drive will fail, just a matter of time, as is the case with an SSD. The difference is that until it happens, the SSD is faster and more reliable. I keep a single 250 GB drive as a working drive for projects, and every so often I store off my project on other (alternate project or backup) drives.
One can argue for and against all day, and in the end one has to go ones own route, which depends on many factors. I can of course only speak from my own experience. From it I say, if you can afford it, try SSD!