No Aga branded stuff is essential, nice to have stackable pans but the prices are eye watering.
I use my Stellar saucepans. Obviously they don't stack but I rarely use more that two in the oven. I can fit three in if I need to.
For grilling I use our old grill pan from my old Neff oven. It doesn't sit on the runners. I put the shelf on the 2nd runners and slide my grill pan onto it, works fine.
Should get a gauntlet if only for reaching into the back of the oven but mostly I like using tea towels so i just take a bit extra care reaching in.
I use two sheets of round bacoglide for toast/toasties, frying eggs, pancakes and welsh cakes. Did buy a new pallet knife 😂
Tesco used to sell baking trays that fitted the runners but I think they've discontinued them. I use my pyrex baking trays.
I have a Le Crueset kettle much cheaper than the Aga ones which were £180 when I was looking to buy.
If you bake you'll need a cold plain shelf.
The only real essential is a timer, there are no cooking smells with an Aga unless the door is opened. Its easy to out something in and forget about it. One good use for Alexa is she does multiple timers that can be named.