No roof can be "flat"; it must slope at least 1/4" per foot and sometimes more to avoid ponding in later years. A standing seam metal roof that can slope 1" per foot needs to be machine crimped as recommended by the manufacturer. You should already be in contact with a manufacturer's rep to get advice for their system for this design. For a conventionally fastened seam design the min slope is usually 4 in 12. I would use full coverage Ice & Water Shield (Grace only) under any metal roofing system Metal will look better if you can actually see the roofing from any place important. The advantage of EPDM single ply roofing is the seams might be 20 or 30 ft apart instead of 18" and it's cheaper and easier to repair. On top of rigid insulation metal would be supported on spaced strapping or continuous sheathing and fully-adhered EPDM would be supported on a wood fiber board. It generally helps to see the house design since so many aspects of exterior design are determined by appearance. I would be concerned with the roof edges and where the roof drains. It is unwise to design one aspect of a house at a time. I can't believe your architect and builder aren't doing this for you.

Well, yes, it's possible.... but. That large amount of complex valleys, trim, and underlayment, will not be inexpensive. The flat metal is the cheapest part of a metal roof. It is not going to be easy to erase the home's identity and move it to "farmhouse". It has almost nothing in common with a classic farmhouse. It's pretty nice as an updated ranch though.

If it is ordered correctly, there is almost zero waste. You don’t order by the square foot like shingles. The lengths come precut to the exact length needed, and all that needs to happen are some site trimming for angles for hips. We had 32 foot lengths for some of ours. We used the exact number of pieces calculated to be needed, and only had the one length left over that we had ordered in case of damage. It’s the trim pieces that you need to allow a goodly percent for waste. Those things are where all of the dollars are in metal roofing.

We put a standing seam metal roof on a large farmhouse-style new build in metro Atlanta, where it is somewhat rare, and while I can't say it increased the value (no data for that really, there aren't good comps for our home for other reasons), it did sell at 98% of asking price the first week. I have loved it...our home has been extremely energy efficient and on a hot sunny day when the roof is so bright it glows I like to think about all the heat it's reflecting! I'm not sure this will be meaningful for your house, but the one thing I wish I'd known for ours is that the gutters available (half round to match the roof) have been a pita to get right. My husband loves the roof and hates the gutters. I could go on and on, but suffice to say we have consulted at least 3-4 specialists with no fixes. But, our house is a two story on a daylight basement with a much steeper roof line that I'd imagine yours is on a CA ranch. We were told the steeper pitch causes the water to run fast, and the lengths required (up to 50') are super difficult to tilt effectively. Also, the house sits under a gorgeous 150+ yr beech tree so there is a lot of gutter cleaning going on around here. And we couldn't find gutter guards for the half-round gutters. Again, I hope this is all moot for you, but in GA where it can get very rainy we wish we'd had a more effective gutter system from the get-go.