Retro fitting stone cills and cavity closures. Any tips / pointers?

[flanagaj]

I'd like to retro fit stone cills to our 1910 brick cavity property, at the same time as replacing the windows.  What I want to understand is 

a) when installing the stone cill, should I fit a dpc underneath the cill which bridges across the cavity?

b) I was also thinking that at the same time I should install insulated cavity closures to stop cold bridging?

c) hopefully not required, but also retrofit cavity drips trays above the windows?

 

I am very practical and wonder how tricky this would be?