Pitched roof to flat

I'm thinking of buying an art deco house which currently has a tile hip roof with internal gutters. The roof urgently needs replacing...

But instead of replacing the roof, I'm think of adding a small extra room on top and making the roof flat (as per standard art deco design).

I'm pretty sure the house structure can take it as I've had an engineer take a quick look.

What I am wondering though is what happens to the electricals, exhaust fan outlets and such that currently run in the roof space ? And how do you handle insulation with a flat roof ?

I think that the electricals in the house should also be replaced very soon. Should that happen simultaneous with the extension ? Or after ? Or maybe before ?

And where do I start ? Get the engineer back to check the structure, then a builder ? Or a draughtsman / architect ?

So many questions....

Thanks