I agree with your tastes, love these 70/80's styled Omegas. Recently bought this NOS Constellation quartz which I think is a fantastic watch with great design. Reminds me a bit of the Mariner (I) but alot smaller at 33mm wide and 7mm thick. Wears a bit larger though thanks to its integrated bracelet. (a bit tight on me though, could use an extra link piece...)
I think it is important to do your homework before buying these watches, do research on the specific model and try to judge the condition from photos. The cases usually have rectangular shapes and sharp edges that often has been polished away.
Its also difficult to know the condition of these semi-mechanic quartz movements, one will have to rely on the visual impression. Fantastic good qualite movements by the way.
Price wise its hard to say since these wathes are not that popular compared to other vintage pieces. But for £400 you should be able to get a very fine example.