I'm thinking about the same thing.

In our new kitchen plan we will have our cooker in the current unused
chimney stack. I know we need a liner to stop build up of grease causing
a
fire hazard.

How does a liner help prevent a build up of grease?
Do you mean a liner for the whole flue or just for the opening where you
will put the fan.

However I finding it hard to find information online about
which extractor will be suitable. All sites seem to selling fully
integrated (into a cupboard), or with chimney hoods, but we don't need a
hood as we are keeping the chimney breast as a feature. We want a
powerful
one as this will be a living area too.

I plan on using a hood