Tufnol is an excellent engineering plastic, but is much heavier than plywood, and shouldn't be used in large pieces in a vane gear, where weight is a disadvantage. Its lubricity is only moderate, and though it was used for bearings in the Hasler gears, it ought to be kept well greased.
HDPE has excellent lubricity and low water absorption, which makes it very useful for bearings on boats. However, it is not particularly light, strong or stiff. White HDPE has a short life in sunlight, black is always better (black pigment reduces the UV penetration into the heart of the material more than do lighter colours).
Polypropylene has nothing particularly to recommend it for marine use.
My vane turret is made from scraps of plywood, bonded and well coated with epoxy and protected with two coats of two-part polyurethane paint. It has not needed any maintenance in over twelve years. Indeed, my entire boat is made this way. I reckon it to be a low maintenance method of boat construction. Plywood bonded onto a timber frame is good engineering, with a good combination of strength, stiffness and lightness. I recommend that you stay with Bill Belcher's construction method; coat it and paint it well and you won't have any ongoing maintenance concerns. By all means use some small pieces of engineering plastic for bearings. Black HDPE is good, but if you find some black Delrin (acetal resin) or Nylatron (MoS-filled nylon), they are also good.