I think the Mamiya 6 only has 3 lenses. As far as I've been told it's a great camera.
I have a Mamiya 6 with the 3 lenses. Its a good lightweight setup. I'm very hapy with the lens quality, and generally its fairly well built.
I still find my composition is slightly better on an SLR though, think its down to parallax and the harder to judge depth of field.
But as has been mentioned, they're not cheap cameras (Nor is the 50mm lens), so it wouldn't be my recommendation if you want to avoid going overboard.
I shoot architecture so I guess our usage will be similar. If you're not doing near / critical focus then a TLR sounds a good option if you want to keep costs down.
I'll throw a suggestion for a mamiya C220 or C330 into the mix. They go for very reasonable prices and there is a good range of lenses. They're also pretty easy to keep running if you're handy with a repairs. (The mechanisms are all simple, and they're fairly large scale by camera standards). My main problem with my C220 has been the lenses tend to suffer with flare rather badly - a hood is essential IMHO and they're less common than the cameras themselves
FWIW this is my little medium format collection:
Rollei SL66, 50+80mm. Best camera when I can lug it, and there is decent light (1/250th minimum handhold speed). Cost inline with hasselblad.
Mamiya 6, 50+75+150, This is my travel camera (Holidays, day out by train). Expensive for what it is, but you'd struggle to find anything better if you want 6x6 and want to keep the weight down and not attract too much attension
Mamiya C220, 65mm, 80mm. Well built, but the lenses are older and coatings are poorer (IMHO), Still plenty sharp. Tend to use this somewhere I don't want to take the above cameras - Eg the beach where I might get sand into it, iffy housing estates.
(Not boasting, just saying it's one of the few camera bodies of which I could actually justify having two, based on usage. The other is the rb) 
