Not sure it there would be any cost savings, but you might consider a copper roof. The oxidized patina would be in keeping with the style of your Tudor.

First of all a flat roof isn't suppose to be completely flat, it shouldn't pond water. Modified Bitumen Membranes are common for low sloped roofs, applied with a torch or hot mopped. Some come with granules on the surface for UV protection, others have an aluminum or elastomeric coating applied to them. A cheap DIY alternative is 90lb roll roofing but it is just that, cheap, and it won't last very long. As far as insulation there is ISO board in different thicknesses that can be applied to the roof deck with hot asphalt, to which the Modified Bitumen Membrane is applied to it. Using insulation would require a larger, custom made eave flashing to cover the insulation and a portion of the fascia board.

pre finished hardy be very care full with who you choose to install the siding it scrathes very easy and it takes very care full handling. i've had to repaint two of those pre finished siding jobs in the last year because of poor crafstmanship by the installers just food for thought. also with azek you can never paint it dark it will peel it self awy from your home the hot months.

I think cedar would probably look the best and over time weather to match the environment.