Ok, right off the bat, if you have trouble seeing and focusing then find a good autofocus camera that you can work with. Nothing else is going to be as good. I love manual focus but as I age I am absolutely flabbergasted at how sharp the photographs are when I use my Pentax PZ1p, and that camera is not one that you will find listed with the top as far as autofocus is concerned.
However if you, like me, insist on continuing to fool yourself into thinking you can actually focus a manual focus camera then the Pentax LX probably has the best viewfinder of any manual focus camera I have ever used, and I have used a bunch including Nikon, Canon, Minolta, Olympus, etc.
Next, though a fast lens (f/1.4) is a must for getting a clear view when you are focusing, don't ever, ever try to shoot anything at that aperture unless you are interested in proving to yourself just how bad your eyesight really is. Do yourself and everyone else who has to look at your photographs a favor and stop down. F/4 is a good starting point though you really should start living by the little adage "f/8 and be there!"
Finally, if you MUST use a manual focus camera, get rid of your SLR and move to a really good rangefinder. Obviously Leica makes some good ones (they are kind of known for that kind of thing) but the Zeiss Ikon is by far the best for those of us with questionable eyesight.
Once more I will repeat what I already said, bite the bullet and buy an autofocus camera you can live with. Even the worst of these will probably be better than anything you will be able to do a a clear, bright day outdoors.
Now, where did I put that Vivitar V3800n slr again??
EDIT - Of course a nice little 8x10 field camera with a good ground glass and a clear magnifier may also be a good option...