Have you tried a TLR? Other than the SLRs you mention, these are the cameras that come with waist-level finders. I use a Rolleicord Vb which is quite small and light. It's all black and therefore discreet, and can be hand held at very low speeds. Mine cost 105GBP 18 months ago. There are plenty around in the UK. It was made into the seventies, so there are 'recent' examples available. I dont think there are many affordable f2.8 options for 6x6. The 2.8 Rolleis are certainly expensive.