F/1.2 is HARDER to focus, not easier. The narrower depth of field is what makes it more difficult. As for the brightness of the viewfinder, anything brighter than f/2.8 will focus fine and you won't notice much difference between the lenses in your range.
If you're planning on shooting only 50mm lenses, get a camera with a viewfinder that shows more of the final picture area and good eye relief. If you wear glasses, you'll want to take into account the eye relief more than if you don't wear them. This allows you to see the entire viewfinder from farther away from the eyepiece.
Any of the Nikon F series cameras would do you well: F, F2, F3, F4, F5.
The Pentax LX or MX are great.
Canon F1 and F1n are amazing tanks.
Olympus OM-1/2 are great.
Decide on what optics you want to shoot with since there are more varieties of 50mm lens than you can shake a stick at and many of them offer subtly different looks to the way they draw. I can certainly tell the difference between my Nikon 50mm f/1.2 and my f/2.0 lenses at f/2 and f/2.8. Smaller apertures equalize the lenses more.
The Pentax K mount cameras give you a flexibility to use both K and M42 mount with a little screw on adapter. This opens up a world of lens possibilities that are usually pretty inexpensive if not free, in the 50mm range. The Super-Multi-Coated Takumar 50mm f/1.4 is one of the greatest lenses ever, made by any manufacturer and comparing one of those to a 50mm Leica Summilux, the Takumar is almost free for a lens that has very similar specs on paper. Each lens line will have its adherents though. 50mm lenses are cheap when compared to telephotos and fast wide angles, so trying out new optics isn't nearly as pricey with a 50mm as it is with a 20/21mm.
Have fun.
Phil Forrest