You can totally get your own ram from OWC and save yourself a few coins. Nothing wrong with that. I think you’ll be pleased with the 5K iMac performance and won’t need a new Mac Pro. Very unlikely you’ll ever have any real world use for anything more than TB2 speeds. TB2 is very useful for things like getting 4K HDMI output of an iMac via a Belkin dock. If your RAID 1 TB’s still work, don’t worry about any additional enclosures.
I wouldn’t recommend buying the Highpoint box. I’ve had annoyances w/ that company.
Bob Zelin recommends Hitachi Deskstars. They even go up to 6TB if you want them. I assume your RAID TB1 enclosure is a Promise Pegasus or similar. They’ll have specs on what drives their systems can handle (mine still kicks butt to this day on my home iMac).
Don’t go Fusion – get an SSD 1TB. Get the most expensive GPU Apple offers. If you want a dual monitor setup, use an extra Thunderbolt Display and tether it to the iMac. It’ll look something like this:

This system is using:
iMac 5k, 1TB SSD, everything maxed out
Areca TB2 Array w/ 4TB Hitachi Deskstars (32 TB)
Bose Companion USB Speaker system
Apple Cinema Display (left)
Belkin TB2 Dock w/ HDMI out connected to a 50″ LG 4k tv.
Black magic mini-monitor w HDMI and SDI out
Promise TB2 10GbE connection to 128 TB network server (speeds approx 800MBps r/w)
Casio 88 weight key piano connected via USB
iPad Pro (awesome to make a list of client notes with checkboxes via ‘notes’ app.)
And for fun, the back light is a Satechi LED bluetooth control RGB light strip.
I have zero need for a Mac Pro and I wouldn’t hold my breath on USB-C on a Mac Pro update. Everything you want is covered with USB3 and TB2.
That help?
John Davidson | President / Creative Director | Magic Feather Inc.