HX Sandman - What's it worth ?

[Innuendo]

I'm kind of against this idea because I'm trying to keep the car as original as possible.

100% it was born a Van and putting back to one is no big deal. This will give you greatest satisfaction and dollar return. You're far better off with a replacement turret over a replacement body for originality.
Your purchase all those years ago is a total bargain by todays standards. Good top tailgates are getting near that price today!

What is different in a HQ turret to any other turret, bar the gills on the side of the early one? What is different in the ventilation in the turret?

There is internal structure on the side vented HQ and people like your mate Byron would be all over it if spotted. Water escape channel, top breathing etc. It could all be removed.