My thoughts as a Mac bigot and IT Consultant (higher ed, Mac labs):
1) Websites such as these tend to be high priced, based on my research. If you finally decide on such a system, try to find a better option (private party, corporate discard, etc.).
2) I would be very concerned about lack of replacement parts on EOL equipment. Only the used market will save you there. In time this thing will die and you will have a nice metal box.
3) Awhile back a colleague actually gave me a 2007 Mac Pro that he was tired of fighting with (numerous trips to Apple for replacement parts, etc.). For that price I was glad to play with it. It was fun to tinker with and fast, of course, but I quickly learned there was a bug in certain Pro's that initiated random restarts (not cool...). Some online said it was the motherboard, some said the RAM, some said dusty GPU vents, etc. etc. I ended up selling it on eBay with full disclosure but having it returned anyway. In the end I parted it out and sold the pieces on eBay. I personally would not buy any of these silver models for more than a few hundred dollars, even though they are "worth" more.
4) Another colleague sold me his 2011 build Snow Leopard Hackintosh for $100 + some trade items (old lenses and such, and a working G5 tower). It is a G5 case with an i7 processor, Gigabyte motherboard, Zotec GPU, 650 watt power supply, etc. I added a bit more RAM to make it 12 and have it triple-booting (OS X El Capitan latest, WIN10 and elementary OS Linux). I dare say this is the one of the fastest computers I have ever used, even with slower HDDs, and compares favorably to a 2012 fusion iMac which is otherwise the best Mac I have in the office.
My point being, you can build your own modern high-spec box for way less than buying a used or new Mac. If the Hackintosh part doesn't interest you, WIN10 isn't all that horrible and Adobe apps look exactly the same anyway.
5) OP mentioned he is running an upgraded MacMini. I am writing this from a 2010 MacBook Pro with SSD upgrade, and I agree, it feels great! But RAM is stuck at 8 gigs and GPU is what it is. Better to put money into newer hardware, unless you can get CHEAP older hardware to upgrade.