I am at something of a loss why a standard DSLR finder cannot do what you want. A decent model will give you 100 percent of the image, allow focusing on chosen areas, and allow you to see distracting elements. If your eyesight is not up to the viewfinder, live view can help with detail and more precise focusing, though it can be a problem in bright daylight without a hood. You can also often get a magnifier for the viewfinder, though it makes it harder to see the whole image at once.
I would second MegaZ's suggestion that you try renting a good DSLR or mirrorless camera, to see whether it can fulfill the requirement of "decent." I'm certainly no great photographer, but I think even my lowly D7100 (a 7 year old DX camera rather than full frame) is capable of decent landscape photography. I can see the entire image, compose, focus and spot meter where I wish, and so forth. If the landscape is there, and the idiot behind the viewfinder sees what is to be seen, the camera can get it.
PS, like Racer above, I had a low-end Nikon, a D3200, and even that could get pretty darned nice landscapes if I aimed it at the right place.