0.58x viewfinder is nice for 28mm, I've looked through it once, but because they are quite rare I was not able to find one in my price range.
You should be aware that only M6TTL, M7 and MP have been made with this type and only MP has the classic body size and shutter speed dial direction. That narrows the selection considerably.
You could use the entire M2 window as a guide, the 28mm framelines are at the very edge.
However I think the external viewfinder is good choice (depending on your style) - and depending on the size of the lens you'll be getting it might be the only choice. Large 28mm lens can block a large part of the built-in viewfinder even if it's 0.58x.