I would suggest 2 cameras.
A TLR for general purpose work, including some macro work, plus an SLR oriented toward macro.
A Mamiya C series TLR with a 65mm and 135mm lens set will do most of the work. If you add a paramender, it works well for close focus up to about 0.8 life size (with the 65mm lens).
6x6 in the SLR world means, most likely, either Hasselblad or Bronica. It will cost you a bit to outfit them for macro work, but they are wonderful cameras. Hasselblad will most likekly be serviceable for longer.
If you consider 645, that would expand your SLR choices to Mamiya, Pentax or Contax. Mamiya or Pentax can be had fairly inexpensively with close focus lenses + extension tubes.
If you expand your choice to 6x7, the Mamiya RB67 is cheap, easy to use and has built in close focus capabilities with most lenses. It is very big, however.
I am a Mamiya user - C330 TLR, 645 Pro and RB67.
The attached was shot with the RB67 and the 140mm macro lens (+ the #1 extension tube - maybe ). The non-macro lenses like the 65mm or 90mm will give you higher magnifications, at the expense of some flat-field performance.