I have just fitted a 25kw Rapid, should be renamed 'massive' on the plus side nice and roomy inside.
Because the flue is now central and the condense runs at the back of the hx cast into the aluminium, the flue is very far back! I had a big space to brick up so was ok, but I think trying to fit into a cored hole without the rear spacing bracket would be interesting!
the internal expansion is easy to get at, it is clipped on a pivot on the left, BUT if you are tight on the left side it could be awkward.
the prv is easy as its internally piped into the condense.
and yes the condense trap is a much smaller screw on/off, easy.