I'm pretty happy with my Moskva-5, though it's a little bulky for 6x6 if I put the mask in; it's also hard to hand hold steadily. There are a number of very nice Zeiss-Ikon 6x6 and 6x4.5 folders with excellent lenses and coupled RF; they're capable of producing excellent images and will fit in a suit coat pocket. Better, they can be had in freshly serviced condition for under $300, sometimes less, or can be purchased in "servicing needed" condition and brought up to snuff for even less, with a little care.

I doubt there's a lens you can buy for anything like a Mamiya that's a lot better than a Tessar, unless you enlarge to a huge degree -- and with a folder, compact light meter, and two or three rolls of film in a pocket, you're prepared at all times with hardly a bulge.