more HDs then I would go for a 600W unit.
I disagree. If you do not want to plug in on of these monster graphic cards you probably never will draw more than 200W. Problem with these 'high' power models is they wast lot more energy in the normal use case - a eCS system including HD probably will not take much more than 20-30W most of the time. Estimated, have to measure in the next days. Additional HD less than 10W.
I use a 350W Enermax as the 300W is not available anymore. Instead AMD I've Intel Haswell (not sure if AMD support in ACPI is at same level as for INTEL), instead WD green WD black (performance and durability reasons), instead Crucial Samsung 850 PRO (same reasons as for HD), instead MSI Asus (I wanted the additional Display Port), Kingston instead G Skills RipjawsX. But that's all personal preference.
COM Port - although my Asus does have one I can't get it to work with debug Kernel. Not sure if it's my fault setting up debug kernel or something is not initialized in BIOS. It works when eCS is booted of course. Let us know if it works for you.