EamoDee :
SFF (The one in the middle)
The good news is that the motherboard seems to use a standard 24Pin ATX connector and 4 Pin CPU power... which helps.
I'm basing that on this thread - which is hardly the most reputable source in the world, but it **seems** to be the case:
https://forums.lenovo.com/t5/ThinkCentre-A-E-M-S-Series/M91p-SFF-power-supply-upgrade-what-do-I-need-to-know/td-p/3603332
The bad news is that the SFF units use a TFX power supply, which while still standard (good news), aren't really built for what you want to do.
Here's a Newegg list limited to TFX PSUs with 400W or more and one 6 Pin PCIe connector:
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=TFX+power+supply&ignorear=0&N=600479293%20600036427&isNodeId=1
There's about 10 of them, but none have an 80Plus certification, and I don't know the actual manufacturer for any of them. They might be fine, they might not, I really don't know.
Also, if you read the thread above you'll see that the person who managed the upgrade had problems with the standard SATA power connectors, the original were 90 degree ones, and having straight connectors were too long. You could probably rig something up, but 90 degree connectors would help.
Finally - are you sure you can fit a 1060 in that SFF case? You'd want to do some very careful measuring before laying out all your money. It doesn't look tall enough for a full sized card from the photo I saw, and while using a riser cable is fine in theory, it I can't see how there's going to be enough space for a full sized card in there. You've obviously got a 750ti... but is that single slot? Or half height?
Anyway, measure carefully!