I've had a waist level viewfinder attachment for a Nikon, and I've also used medium format gear (like the Yashica) that used them, and I found that I didn't like them on 35mm that much.
The 35mm viewing screen feels much smaller than square medium format, making it difficult to use without flipping up the magnifier and holding it up to your eye (which reduces the benefits of having a "waist level" finder).
Also, with square medium format you don't have to change the camera orientation switching from horizontal to vertical shots, but you do with the rectangular 35mm format. I found holding the camera to make a vertical shot while using the waist level finder to be very cumbersome (the image is reversed left to right so it is harder to aim and get level, you have to hold the camera funny, etc).
For 35mm a waist level finder is great for low level shots and maybe macro work, but that is about all I found it practical for.
Personally, I'd go with the Yashica if you really want a waist level (I had out a Maymia TLR, a C220 if I remember correctly, nice machine).