For Photo's work, you don't need the 8 cores machine. In fact, due to the clock speed usually is lower on the dual CPU model, you photography software's performance may be a bit lower than the single CPU model.
For photography work, the 5,1 still a capable machine, but TBH, not as good as the new iMac. Which has a very nice 5K display, fastest CPU in all the Mac, that's what you need in photo's work (plus some decent amount of memory, of course).
IMO, it didn't lack of any important thing, unless you need thunderbolt for some reason. The biggest limitation in the cMP now is actually the CPU (more precise, the CPU single core performance). If that's fast enough for you, then you should be fine.
If you are budget tight, you should consider the single CPU 4,1, but not dual CPU 5,1. The 4,1 can be flash to 5,1 for free. And then a W3690 (or W3680 / X5677 if that's significantly cheaper in your town) will make it the fastest photoshop cMP.
I never use the M6e, so no comment on it. But yes, you need a SSD to make it run smoothly. The best CP ratio option is just buy a SATA SSD (e.g. 850Evo) and then plug that into one of the SATA slot (e.g. the lower optical bay), for small files read / write, the high PCIe bandwidth usually won't help anything, SATA 2 is good enough for most of the job (unless you deal with multi GB size photos everyday, then the PCIe SSD may able to speed up your workflow).
I don't know how much that will cause in Europe, but what you are looking for should be something like
1) Mac Pro 4,1 (Single CPU model), which can cost as low as $400.
2) RAM, start with 3x8G DDR3 RAM is recommended, should be well below $100. You may go for 4x8G or even 3x16G. However, DO NOT go for 4x16G if you are using W36xx CPU (It won't boot!).
3) CPU, W3680 usually is the best CP ratio option, as low as $150 (but seems the price went up a bit in the last months). And all X5677, X5680, X5690, W3690 are good CPU upgrade.
4) A 1TB 850 Evo, $300 (or 500GB if budget tight)
5) May need a GPU upgrade, a used 7950 can cost as low as $100, and able to self flash to be the Mac graphic card. Even though you don't flash it, it still work OOTB, but just no boot screen.
6) USB 3.0 card is a useful upgrade as well, Inateck KT4004 is a good option, around $25
7) few multi TB HDD as you wish
This should cost much less then the dual CPU 5,1, and actually perform better than that for photo's work.
From memory, I never see any photography software use more than 6 cores (may be not even 4), even it does, it's only a matter of seconds, and should be only able to do that very occasionally. There is no point to buy a dual CPU model for much more money and let the cores stay idle for 99% of the time, but not buy a single CPU model with faster single core speed which is really useful in most of the work.
Also, once you go for the dual CPU option, the cheaper W36xx CPU upgrade is not your options anymore. That's another disadvantage (if you don't really need more than 6 cores).