Whatever storage solution I buy is going to serve as my backup, so I'd prefer RAID I think for peace of mind. I've even considered something like a WD MyBook Duo and converting it to RAID 1, although I'm not sure how convenient that would be if a drive failed.
Considering the above reply to this...
RAID 1, 10, 5, and 6, can protect data on the NAS from detectable drive or drive controller related data errors. With common gear, they happen (with any, they happen, but the real-world difference in chance of any can be multiple orders of magnitude). With a turnkey NAS like a Synology, other error sources are not detected, much less correctable (but a Linux or FreeBSD software RAID, using ZFS or BTRFS, is its own can of worms, as is MS' ReFS--good, but not remotely turn-key).
The added drives in RAID do not make for a backup. At all. I've personally seen more cases of data destruction via software than hardware, and RAID can only protect against a limited amount of hardware errors (even ZFS and BTRFS are not fool-proof). The NAS itself can be a backup, as in having a copy on the NAS and on your scattered PCs, but RAID has nothing to do with that.
What RAID mainly can save you from is downtime, which means wasting nights and weekends, from having to rebuild the data on the NAS in the event of an HDD failure. For home use, the continued operation of the server tends to be secondary.
I've even considered something like a WD MyBook Duo and converting it to RAID 1, although I'm not sure how convenient that would be if a drive failed.
You could do that, but if you find anything you might want to do other than read a CIFS share, you are likely to find it wanting. Just skimming a few reviews and some user reviews, the My Book Duo looks about like I'd expect.
The home user Synology boxes benefit from having the same software base as their SMB counterparts. Meanwhile, consumer companies like WD, Seagate, Netgear, Trendnet, etc., are going more for price points and marketing bullet points than value. Buffalo should be fine, but they tend to lack a lot in terms of features for your money (in part just due to the likes of Synology and QNAP having a big software and market head-start, and there not being high profit margins). QNAP's UIs tend to be glitchy, and their support lacking, in my limited experience with them. Drobo make good products, but generally are a bad fit, as they focus on business and "prosumer" markets, so tend to be missing a lot of features you might want (you don't want any of them now, but use a NAS for awhile, and you probably will ).