Just to further elaborate on Chris' post above. The S5, S6, T6, C31, D31, E31, E32 (Airedale), and H32 (Mastif) are all very much the same ilk as the D32 (Greyhound), in capacities from 500 to 750cc. To a certain extent, the Wessex, Powerflow, Z, and Z1 of 1934/35 are a continuation of these, but they started to evolve into the subsequent models with loss of the finned oil sump, more bulbous petrol tank, and alterations to the frame.
The 500 and 600cc Aero models are not the same, but were the replacement models that superseded the above. This new pattern of lighter, large capacity side valve began with the Blue Chief model of 1935. These are just as pleasent machines in their own right, and to a mechanical extent better as they reverted back to a simpler valve actuation method.
The C32 is one of the small 350cc engines punched out to 500cc, and mounted in the single loop frame of the lightweight A31 models. So an interesting hybrid machine which I for one would not mind having, but really a lightweight machine and nowhere near as substantial as one of the 'proper' big twins.
The Endeavour is a completely different animal altogether, transverse engine, shaft drive, and commanding premium prices even when new. It was a ground-breaking model when introduced, did not sell very well, and still retains a mystique to this day. Hence the prices asked, but it is all about perception as there are may other side-valve Douglas models that are far rarer. Such as the Greyhound and Mastif in 1933 (D33, H33) or even the Blue Chief if you want that later, sumpless look.
So if you want a big sump model, any listed in the first paragraph will do it for you. If you want something more lithe in appearance, then the Aero models are a good choice. Either will give you that big-twin Douglas experience, and once mechanically sorted give minimal bother. While sidecars were offered for the Aero models, sidecar mounting points are built into the frames of the Greyhound variants. Something to keep in the back of your mind if you feel a sidecar is in your future.
Good luck shopping!
-Doug
