If you already have a good light box/ table you can pretty much use anything your comfortable with an gives you enough eye relief that you want. If you like it close 8x and up are fine even the cheaply Agfa versions are sharp enough in the center. If you would like to evaluate the whole of the negative you need the 3x medium format loupes. Or you can get a large handheld magnifier they can be found anywhere. I bought at the discount store a fold out magnifier I think intended for thread counting or measuring, it sits on its own fold out case, but the glass is terrible especially towards the sides and is not that high of a magnification. I also use an interesting product called a magnabrite. It's a spherical style magnifier you place right against the negative and is quite nice. They make large versions too. I have a busted 50mm 1.4 lens that I used to use as well.

Your 50mm 1.2 would probably be great, I have the same lens, but I wouldn't risk using it as a loupe with its huge rear element constantly exposed/ handled.