For a modern camera, the Fuji GW690III is completely solid and quite rugged, with nothing rattling or gimmicky... it is a metal body sheathed in engineered plastic, I'd say the build quality is more or less on par with a Nikon D300 body. The image quality is arguably the best possible in the format, very sharp, contrasty, and I found even the bokeh to be pleasing. I bought near mint ones for $700, but beware of the heavily used ones on eBay.
They are noticeably more solid that the Mamiya rangefinders, which I think are really poorly designed, regardless of plastic/metal issues. They don't break down so much as irritate....
Otherwise, that new Fuji-Voightlander 6x7 folder seems to be the class leader but it pushes your budget.
Older German folders can be very nice too but I'll defer to experts. I think the front struts are the weak points for most of them.
Frankly, none of these are close to being Leica quality build-wise. I guess if they were we could never afford them. But they are very utilitarian and quite practical picture making machines and a 6x7 or 6x9 negative blows anything smaller away.