does anyone know of a rear brake caliper that has both a hydraulic and mechanical brake, that will fit straight onto or, with a small amount of adaption onto a 04 bombardier ds 650, appreciate any advice. thanks  

I am not aware of a caliper that is both mechanical and hydraulic. I don't think they make one.

in the years past i have retrofitted hydraulic brake systems from sport bikes,  they seem to work very well with little modification to fit the atv.
the main modification i have to make was to add to the mounting bracket on the atv so it would bolt up straight.

not hard to do just a welder a drill and some good 3/8 inch steel.  

Go to http://www.quadtech.co.uk/. I have purchased the very thing as it is required in UK for M.O.T.