"Friend of a friend" came by yesterday, and had a gander at the trailer - the short version re whether it's worth sorting it out is "...well, maybe, depending..." It'd need the solid axle removed, a leaf spring kit put under it, and assorted welding adjustments to accommodate mounting that. He also wasn't fussy about the axle stubs used - they're from the rear wheels of a front-drive car - but they can at least be unbolted and replaced - chucking a spare hub in the toolchest would take care of on-the-road failures. The springs and mounting hardware would cost about $100 Cdn., which is doable if it means we can use the existing axle, hubs and wheels...
If we can't, it gets prohibitive - a 3500 lb. axle runs about $225 more, and then there's wheels and tires to consider, not to mention labour - probably way better to sell this one and look around for something that's ready to roll. Since the basic 8x5 design is set, we can go ahead with some prefab work - build the sidewall, front, roof and rear panels - and work on the floor/base and how it'll be attached to the trailer.
(Just to put the icing on the cake, pack ice has blown into the bay, locked solid to the coast as far as the eye can see, with a nice 20 mph nor'easter blowing onshore after a hundred miles of moving over the icefields - you can literally smell the ice when you step outside, and it carries a very special sort of damp clammy cold that goes right to your bones...that's what you get for living on an island in the middle of the North Atlantic..)
Anyway, thanks for all the advice and encouragement thru the posts and the PMs. We'll keep you posted...