For fastest external storage, this TB2 PCIe external box is a tad under $200:
https://www.amazon.com/Akitio-Thunder2- ... B00LTAUTHE
Appears to support NVMe PCIe SSD storage under mac OS10.13 (High Sierra) which would mean readily available fast external storage...
https://www.anandtech.com/show/9799/best-ssds
1TB "entry level" PCIe SSD $339, so your looking at a little more than $500 for 1TB including the enclosure. Samsung 860PCIe will hit your wallet harder.
On the other end of the affordability spectrum, there is this for SATA SSDs:
https://www.amazon.com/Sabrent-Tool-fre ... +USB3+case
I note one comment indicates "supports Trim". Under mac OS, I'm not sure.
If budget is an issue, I would suggest considering using the internal Mac Pro SSD as your boot drive and, like MLC has recently done, offloading data to more affordable external storage. I recently picked up a new 1TB Samsung 860EVO SSD on eBay for under $255 (broken out from bulk purchase by seller), so SSD storage is certainly quite affordable.
So what do you need for boot? The base 256GB drive is enough if you manage storage carefully. You could even keep a couple of current projects on the internal if you don't fill it up with photos, iTunes, and emails.
Inexpensive external spinners are great for daily project backup and Time Machine backups. I am now using the 8TB Seagate Archive Drive, which is available for $150 in an enclosure:
https://www.amazon.com/Seagate-Expansio ... hard+drive
FYI, these drives cost more if purchased
without the USB3 enclosure.
When a drive fills up, I put it in storage and start with a fresh one. Actually, two, as I rotate two externals for Time Machine use.
You might want to consider having a daily manual Project Backup spinner in addition to your automatic Time Machine backups. With a Project Backup always available and storing loads of stuff, it is easy to find old projects when the client calls for an update/rework.