Need some HDX upgrade advice

[innerbooty]

Hi,
I posted previously related to some issues with an Ebay HDX purchase that went south. I want to give the upgrade another go, and just had a few more general questions about doing a methodical, careful upgrade:

1. Computer - I just finished flashing a 2009 Mac Pro 8-core to 5,1 and now have the SSD drive upgraded to High Sierra. Are there issues I would need to be aware of relating to running HDX on this system?

2. Pro Tools - I am inclined to at least initially sign up for the monthly subscription plan for the (gulp!) $80/month. Does this make sense? High Sierra is as high as I can go on my Mac Pro. Wondering how long until that OS is no longer supported? Or, if there is a previous, rock solid recommended version of PT that I can get a Perpetual license for, does that make more sense?

3. HDX Card - I plan to buy a used HDX card. I initially will be continuing to use my Apogee AD/DA-16Xs with X-HD cards. Will I be able to run DigiTest on the HDX card to verify it works while using the Apogees as my interface, or will I need to have an Avid interface connected? Do I need to be concerned about the age/vintage of the HDX card?

4. Interface - Wondering if the HD I/O would be a significant sonic upgrade to the Apogee AD/DA-16X's. I like them, but they are getting long in the tooth. I want to make sure I'm optimizing the sonic upgrade of going to HDX.

Thanks in advance for any thoughts!