A few points here. I like the 6x6 format too- mostly use Bronicas though I do have a mamiya 7ii also for when a slr is just too heavy/bulky/conspicuous or where a square approach just isn't right, or when I need to handhold, especially at low speeds.
The first is the fact that the Mamiya 6 is a quite elderly camera, discontinued some years ago now. They are as people have indicated, highly rated, but you have to expect that any you find will have done a lot of miles, and you have to know how you're going to get it fixed and get spares. Mamiyas can be quite expensive to mend.
The second is the fact its a rangefinder. That means benefits in terms of portability and formidable sharpness. But these things don't come free, and you need to be sure that you really do understand the implications of using a rangefinder rather than a slr. You know, the no long lenses, no close focus, no tight headshots, more difficult to use polarisers and very hard to use grads; focussing that some people find great, others especially if used to autofocus, might find kind of hard to get used to. A bit fiddly to change lenses; Primitive metering by comparison with a 35mm slr and where you really will have to learn how to interpret what it says rather than set it to matrix or whatever. None of these are reasons not to get a rangefinder- they're things you need to understand before you do.
Meanwhile there are prisms for Hasselblads and Bronicas that give a laterally correct image in the finder as well as ( or without) metering. There are grips that transform the handling certainly for Bronica and I imagine for Hasselblad too. You might yet find that you're misjudging the slr's capabilites.