Tough choice Velo, but i'd be reluctant to fork out £675 on a new box with a bad reputation, i've never heard of a gearbox to be non-rebuildable though, a good gearbox specialist should be able to do it and offer a guarantee.
Just did a Google search on the Comex box, noticed its a CVT, Ford tried CVT years ago without much success, but then the old 70's DAF's used CVT and those were bombproof.