I'm thinking of going with E-M1 Mk2 + 12-100mm f/4 to start with. Then a fast prime for bad light/indoors.
I think that's a good option.
Would you get the Sigma 30mm f/1.4 DC DN Contemporary Lens OR Panasonic Leica DG Summilux 25mm f/1.4 OR Olympus M.Zuiko Digital ED 25mm f/1.2 PRO? How much of a stop difference is there between f/1.2 and f/1.4?
In terms of aperture and impact on depth of field, there's about a third of a stop difference between f/1.2 and f/1.4. If you look at
DxOMark's lens database, focusing purely on the transmission measurements, the m.Zuiko 25mm f/1.2 and the Leica DG Summilux 25mm f/1.4 are both T1.8 - that is, they should offer the same exposure (light transmission). They measured the Sigma 30mm f/1.4 as having T1.7, so slightly better transmission (although that's on µ4/3; when mounted on Sony, it's still T1.8, so hard to say whether it really gives you any benefit or whether it's just a measurement variation).
I don't know much about the Sigma. For the m.Zuiko versus the Leica DG, it largely comes down to what additional features are important to you. The Leica is smaller and lighter, but lacks weather sealing. Reports I had read indicated that it might be sharper in the center, but it falls apart in the edges pretty badly and also has a fair bit of distortion. However, the Leica is also significantly cheaper. (As a disclaimer, I have the old 4/3 Leica D Summilux 25mm f/1.4, as well as the m.Zuiko 25mm f/1.2. The 4/3 version is superior to the µ4/3 version that we're talking about here. While I think the Summilux is likely technically superior in image quality to the 25mm f/1.2 PRO, I don't use it anymore after getting the 25mm f/1.2.)
I'd also question whether the 25mm or 30mm focal lengths are what you want. As you're probably aware, there's a 2x multiplier when figuring the similar angle of view with your Canon full-frame setup. 25mm looks like 50mm on your Canon, and 30mm looks like 60mm on your Canon. Even though I started with the 25mm focal lengths, I find them a bit tight for indoor shooting.
Part of the reason that's important is because the 25mm lenses are surprisingly some of the weaker primes on the system, and you do have wide-aperture options at lower focal lengths that perform better. For Olympus, the 25mm f/1.2 actually does not score as highly on pure sharpness testing as the 12-100mm f/4, if you can believe that. It's really a testament to how good the 12-100mm is, but particularly when you compare the 25mm f/1.2 with the 17mm f/1.2 and 45mm f/1.2 primes - the sharpest and second sharpest lenses on the entire µ4/3 system at the moment, respectively - it shows that the 25mm f/1.2 probably could have been a lot stronger. That's not to say it's a bad lens, of course. I think it has "character" and really cherish many of the photos I've taken with it.
If you like the idea of a wider angle of view, the two "ultimate" choices you have are the m.Zuiko 17mm f/1.2 PRO (my personal favorite, most-used lens on the system) and the Leica DG 15mm f/1.7 Summilux. There are also the m.Zuiko 17mm f/1.8 and Lumix G 20mm f/1.7 pancake. With the exception of the m.Zuiko 17mm f/1.2 PRO, all of those choices are considered highly portable, small, and light, although the 20mm pancake uses older autofocus motors and is regarded as being slow and noisy by comparison. Also with the exception of the 17mm f/1.2 PRO, all of those lenses are T2.0, so just slightly worse than your f/1.4 options. (DxOMark doesn't have a lens entry for the 17mm f/1.2, so it's unknown what its light transmission is.). The 15mm f/1.7 is very highly regarded by those who use it, although I haven't seen it compared directly against the 17mm f/1.2 PRO. Much of what could be said in the comparison between the 25mm primes applies here, as well, except that the optical quality of the 15mm f/1.7 seems to be better than the 25mm f/1.4 (or perhaps, more consistent).
