Quick question for those of you with 6,1 Mac Pro's already - I'm seeing a lot of 256gb HD MPro's on eBay (together with D300 video cards), I'm assuming I should steer clear of those as the extra space will be needed for apps and OS right? 512gb or 1tb minimum?
I will break down my thoughts on buying a Mac Pro for you: I estimate a used price, I would pay at most, based on the prices I saw on ebay in Europe (private sales). This is for a base model with 256gb SSD and up to 16 gb of ram. The 4-core usually always has the D300 - all other models should at least have the D500. If that graphic card works, you are lucky! But I told you, there are many people who had bad luck with it and are now stuck with a badly working Mac Pro with a lot of cost to replace the graphic card. So, you can either take that risk or stay out of it ... your choice.
4-core: 1400€
6-core: 1600€
8-core@ 3.0 GHz: 1800€
8-core@ 3.3 GHz: 1900€
10-core/12-core: 2000€
You might find more expensive offers and sometimes people paying more, but that is madness, as I have seen auctions selling for about that kind of money and those processors (6-core to 12-core) can be bought for 200-300€ and are not that hard to change.
From that, you might add up to 350€ for a bigger SSD and maybe 400€ for 64gb of ram. The 128Gb ram kit from OWC can be bought for under 1000€ from alternate. Since High Sierra you can buy Samsung Evo SSDs (up to 2Gb of internal memory, I believe). So, the included SSD should not matter that much, IMO.
If the machine still has a year or two of Apple care, that might be worth a bit more, as you have less risk.
Other thoughts:
Some used Mac pros are already user upgraded - which is more risk, I would say ... Most of the time, that is pretty obvious for Mac Pros with a D300 and more than 4 cores. You can be pretty sure, the CPU has been upgraded by the user or company. For all D500s and D700s there is an Apple exchange program, in case it has problems. As far as I know, it is still valid.
My best bet is the 8-core@ 3.3 GHz the next best thing would be the 10-core and the 8-core@ 3.0 GHz. Before I would consider a 12-core, I would buy a 6-core and maybe upgrade the processor later. Most important to me would be Ram, I would go for 128Gb. I never found a machine above 64 gigs online, so, I actually prefer less ram, as it will be useless to me anyway.
Most of the time, there are 4-cores and 6-cores offered on ebay, that is why it is your best chance to get one of those for the price I estimated. With the 4-cores, you always get a D300 (never saw a 4-core with D500). So, my suggestion is: look for a 6-core with D500. That is a pretty safe bet, can be bought for not to much money and is upgradable in every way: 1.600€ + 1000€ for 128Gb if ram (or about 400€ for 64gb) + 600€ for a 2Tb SSD (you need an adapter, but that can be both really cheap) + 300€ for a new CPU
IMO, this is not REALLY well invested money, as it is really old hardware, but that is just the it is with Apple. For a iMac pro with 128Gb and 2 Tb SSD you will pay almost 10.000€ (vs 4.000€ for this machine). The iMac pro will be a bit better, but also more then two times the price.
Then again ... if you would consider going Windows, for 4.000€ you would get a machine (without monitor) that would outperform the iMac pro. But that is due to the fact, that Windows 10 performs better for audio than Mac. I still like Mac OS, as I am used to it. But the more I think about it, the more I see that I will probably jump for Windows in the end. The chances that Apple will really have better offerings for pro users in the future seems slim to me. At least, I am already using Cubase, so, there is no fear of having to learn a new DAW!
I hope my thoughts help!