Yes I did recommend that to you as you were not open to the idea of NAS at the time. Plenty more websites with stories about why RAID-5 is not a great idea along with woes of recovering data.
If we look at a standard system, I think from memory the drive cage can take 3 drives in total. (I'm not counting those bodged with velcro or blutac to inside or outside of case!). This would seem to be best used with a system drive SSD and two large mechanical drives in a mirror (RAID-1), assuming external NAS storage was not a viable option.
RAID-5 has severe performance degradation with a failed drive within the raidset which lasts until the failed drive is replaced and resynchronisation is fully completed. The rebuild time can be quite a length process and performance degraded until it completes. The read errors that can occur with data corruption due to partial media failure mean the data could become corrupted before a drive failure occurs meaning a recovery from backup is needed.
Most people charge into a RAID-5 setup without fully understanding the limitations of it. RAID is not a form of backup, it provides resilience of the available data. A backup of the data in some form is still required (some firmware upgrades can wipe raidset data).
... gives more available capacity than other RAID levels over same spindle count
... is slower for writes due to parity check calculation and writing data plus parity
... generally has a penalty on writes smaller than the block size. Shrinking the blocksize of the RAID5 device, slows down the RAID5.
... needs prolonged recovery time after failed drive (see above), integrity can be lost should another drive fail before it completes meaning complete loss of data.
In short, I wouldn't touch RAID-5 with a 10 metre cattle prod!
The total amount of available storage will always be exceeded in a shorter time than you think and ultimately the cost will be down to how much you value your data... The DS212 paired with an external disk for backups would be ideal. You pays your money and takes your choice.
Just my opinion on it, I'm sure you all will have your own!