Technically it doesn't matter, lying down is of course also possible, but I would get some relatively large rubber feet from the DIY store, transparent ones, so that you don't hear the vibrations of the plates so clearly.

I strongly advise against the j devices, they are simply rubbish, my recommendation is clearly also for 4 bay, but if it can only be 2 drives, then by all means take something like this, you simply have much more of them and they don't cost much more.

For 2 bay, I would take this one, it has 2GB RAM and a 4 core processor and the disks are easy to install and replace in drawers without tools.

Synology DS218 (0 TB)

If a 4 bay is possible, I would go for the DS418, it is more expensive, but you would also have the option of installing smaller disks to achieve the same storage size, because you have 4 disks at your disposal:

Synology DS418