Tom: congratulations on the Platypus. I'd love to test ride one, set up per all the Rivendellian specifications, too.
Cold and wind: I'm no arctic rider, but I'm fine down to high teens with light wind and no rain; secret? 3 things: (1) wool and layers of wool; (2) add and remove clothing per conditions (I just added the winter Barley/Junior to my 1999 Joe Starck stripped down gofast exactly to carry outer layers as temps warm up), and (3) start out a bit cold; don't worry, you'll warm up in 5 minutes.
My main problem areas in cold weather are ears and fingers. Solutions, imperfect but they help: balaclavas or wool neck gaiters that you can use to keep your neck warm and pull up over your head and ears and wear under a winter cycling cap or helmet; gloves 2 sizes too large; and gloves in layers instead of big fat down-filled arctic-type gloves. Silk liners under wool liners under lightly-lined windstop gloves -- I use ex-German Polezei leather gloves with light fluffy lining and snap wrist straps in XL over L woolies and M (my real size) silk liners.
Loads and handling: Rivendell brought Carradice saddlebags back to the US market and later developed with the Sackville line what I believe are truly the best saddlebags on the market. Why do so many people want front loading on their Rivendells?