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That’s a really useful link, thank you!
The tl:dr appears to be that 2010+ is better as it’s already 5.1, but the 2009 with 4.1 I currently have in my possession can be flashed to 5.1 easily and then it’s pretty much the same. I can then look in to any hardware upgrades in the new year. There’s already plenty of ram and something like a total of 4tb of storage in there, which is great for PS/AE scratch disk.
edit: there's one of these up for grabs so I'll go with that one. Faster and already has 5.1 firmware.
older one's still run fine you can get RAM for smallish beer, and if you want to can upgrade the processor, you'll need to get one which is 5.1 so it'll run upto high sierra without any issues, if you want to run mojave, you'll have to upgrade the graphics card.
This guide should help you fill in the blanks, bit dense but lots of info
http://blog.greggant.com/posts/2018/05/07/definitive-mac-pro-upgrade-guide.html