May I ask why HDD instead of SSD?
HDD is more cost effective than SSD in terms of per gb costs. While it does run a bit louder, Bitcoin nodes has a lot of I/O activity with it due to the constant fetching of block info to supply to the nodes. Flash storage can degrade over time with constant reading and writing. HDD is more reliable in this aspect.
Additionally, because MyNodeBTC comes preloaded with software, etc., how can we know for certain that it has not been tampered with or a trojan/keylogger, etc. has been installed on the device before it reaches us?
I would recommend for you to run one with Raspberry Pi.
Do not put your files on the MicroSD card if you're going that route. The high I/O will wear out the MicroSD card very quickly. You would want to synchronize your files on a computer before putting it on a Raspberry Pi. It's too underpowered and will take quite some time, perhaps up to a week to synchronize. You will want to use the options to disable wallet and all the unnecessary stuff to try to save the resources and perhaps limit the peer count. RPi is very underpowered for a node but it performs well for it's price, it'll work just fine for a medium sized full node.