Forum member Mike Ellis is highly knowledgeable on the subject of Premier drums. Contact him through the forum.
Now, some answers to your questions from my personal narrow viewpoint and experience:
My general feelings: Think of the most beautiful, shapely, English fashion model / actress "goddess" - one turning heads everywhere she goes, a difficult and demanding celebrity requiring and expecting constant attention and pampering. She hasn't in her nature to be consistent. Every effort satisfying her mystifying temperament makes you deleriously happy just knowing that she's yours! But, a nagging thought won't leave you! Previous lovers, though perhaps not embodiments of physical perfection and elegance, were faithful and true. They willingly returned your affections and efforts to please them, and were for the most part far less demanding of your time and treasure. Still, it is your "goddess" with whom you are obsessed! You never tire gazing at her exquisitely sculptured form and loveliness. You marvel at her uniqueness, her style, her distinctive British heritage, class, and pedigree. With her on your arm the world is your oyster! She makes younger men wish, and older men dream! You're more than willing to continue in this madness, despite the emotional toll, because it's worth the trouble.
Now, if you could just locate a Premier slot head drum key! Hurting
More specifically:
-the shells
No finer European craftmanship to be found on the planet. Exquisite workmanship, design. No detail overlooked, no matter how small.
-sound
Different than most other vintage drums. Clear and round. Superb projection. Snare drums in particular have magnificent response and action.
-hardware
Stunning hardware and chromium plating. Every effort made to design and create "user friendly" hardware for the professional working drummer. Reduced set up and breakdown time thanks to brilliant design R&D. Excess weight eliminated. Pedals lightning fast and effortless.
-tuning
Problematic at times due to the tab loaded "flush braced" tension brackets, because shock and impact are reduced by the tabs holding the threaded insert nuts "floating" freely and "giving" a little when stressed. Internal springs would have solved the problem, but springs would have resonated, and they aren't part of the vintage British drum design concept.
-ability to find parts, etc.
Not too much a problem thanks to the internet. Keep in mind that some hardware parts are made to specific British measurements and most commonly available contemporary screws and mounts won't be compatible.
-some are beech wood and at some point went to Mahogany? I'm assuming the latter sing more in low tuning?
Both choices sing and project. The mahogany more in the middle and low frequencies. The beech are loud, clear, bright.
-heads went from metric to international
I believe the changeover in tooling came around 1968. Personally I find the pre-international drums the more beautiful and better constructed, though purchasing quality heads for them and their die-cast counter hoops can be nightmarish. The international sized drums present no head fitting issues.
-thinking about moving my Gretsch for a one and paying bills.
Can't make the choice for you, but I personally would buy a nice well priced vintage Premier kit first, and then decide if you want to part with your Gretsch. I wish I'd never sold any of the many vintage Gretsch kits I've owned. Neither do I wish to part with either of my Premier kits.