Thanks all for the posts so far.
I agree that building the whole set around a single piece probably isn't the best way forward. I'm basically sold on earlier period as this correlates with my SCA persona (which hasn't been properly fixed in time yet, but between the concerns of my lady and my own, we appear to be looking at somewhere in the 12th to 14th century.
Between that, my limitations as to what I can afford and my dual aims of this armor a) keeping me safe in a BotN context and b) looking good (and being not too heavy) for the SCA context - in which I will, after all, spend the preponderant amount of my time - I think I'm basically set for a Klappvisor Bascinet (which allows me to use different visors or no visor at all if and when I need to adjust), with splinted arms and legs and a coat of plates of some sort.
That - admittedly very basic - description should have me covered for a fairly large span of history in my 'target zone'. Right? Right?
Now, on the splinted bits of armor I'm very lucky because I have someone who is willing to help me, and I just saw a really nice set of legs he made for someone. It's going to be great. But he gave me homework - to find an effigy (or tapestry or painting or whatever) with the sort of kit I want to emulate.
my first attempt at Google Images didn't get me very far (apparently my GoogleFu needs improving). I strongly suspect that there's the odd person or two on this forum who might have a link (or several) to sources where I might find images of historical sources for what a set of armour with splinted arms and legs (and/or a coat of plates) might have looked like? Now I would get some proper books and *really* do my own homework if I wasn't on a lengthy business trip, but these days I barely have time to check my email, and no time (or place) to sit down and read a book - and he does want me to send him an image in a couple of days or so, so I suspect he's okay with it if I utilize what we now call 'networked intelligence'.
So I hope you won't consider this cheating - I do want to get this stuff right. Any and all help, again, much appreciated.