It sounds like a interesting project and I wish you well. I disagree a bit about biplane design and NACA--the old reports are a treasure trove of information--but it's certainly true that biplanes kind of fell off the radar after WWII. Your performance goals seem achievable without any exotic solutions.
If you are interested in using an existing design as your starting point, you might want to take a look at the Javelin Wichawk. In addition to the more common side-by-side two-seat configuration, there was an option for a three-seater with two passengers side-by-side in the front and the pilot behind. There are also a couple of "modern" biplane homebuilts out there like the Durand Mk V and Sorrrell Hyperbipe, but they are not "classic looking."
Another approach would be to take a vintage aircraft of the period you like as a starting point and create a new design with that inspiration, perhaps even doing a 7/8 scale down with some tweaks. Two that come to mind for me are the beautiful taperwing Waco designs like this ARE Aristocrat (imagine a three-seater with a Rotec radial engine!) and the de Havilland DH.87 Fox Moth and the Thruxton Jackaroo cabin conversion of the de Havilland D.H.82 Tiger Moth.
Cheers,
Matthew
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