Mark,
This question is like a motor oil thread. Every opinion is different and most are correct. Protect yourself for battle but not so much that you can't move. You know this from street riding.
Helmet is #1 of importance and expense. I'll run my Leatt "Dirt" helmets and Bell MX9 ADV under 65mph in the dirt or street. My Arai Signet or AGV K5 are for anything over 65mph or 350lbs is what i wear. I could care less what anyone thinks like "That's not an ADV helmet, you should get XYZ cause it is 60grams lighter, has a visor and looks cool". While i might agree my head is safe so thanks for the opinion.
Boots are #2 - Your bike will determine what you need just like a street bike. Motocross boots belong on a MX bike and nowhere else. I use Alpinestars Tech 7 Enduro for my XR400, XR600, KTM 500, KTM 525, KTM 450, DRZ400, KTM 950SE, KTM 1090, ATC185 and more bike that i will buy. I also love my Forma Adventures that will likely be my sole ADV boot.
Chest Armor is #3 for me. Once you have had broken ribs because you chose the wrong chest or back armor is true learning experience. Good thing i broke my ribs when i was 9yo when i forgot to wear my JT Roost Protector. I wear a Leatt 5.5 Pro HD in the dirt with a Kriega backpack for Agua and snacks. I am not a true ADV rider yet but my Astar GP Plus works fine for my big KTM's on the street.
Gloves is #4 for me. I wear Klim, Astar, Dainese, Racer, Fox, RevIt, Cortech in shorts or Gauntlet. My wrist brace dictates what glove i can use and what mood i am in dictates how far up the wrist it goes.
The list can go on and on but like most members on ADV i have a ton of "Extra" gear (Aka i am gear hoarder) that you can try before you buy based on your size of course. More then happy to let you try before you buy because "Riding Gear" is a whole other bank account waiting to be drained.