Short update, mainly to thank you for your opinions and assistance. I've got a Synology 920+ NAS and a couple of 16TB server HDD. Once this is set up and working, I'll decide if I add more RAM and/or a M.2 drive to be used as cache. This unit admits up to 4GB RAM extension, but it's been tested to work well with an additional 16GB DIMM for 20GB RAM.
Main concern is how noisy it's going to be with 2 fans and up to 4 HDD, but I can always replace HDD for SSD units sacrificing capacity. Fortunately it can be placed away from my desk.