How much pasture per turkey?

I'm looking to raise turkeys that are as completely pastured as possible. (I plan to purchase some Bourbon Red & Blue Slate poults.) How much space would the average turkey need in order to fulfill most of its dietary needs and be happy and healthy?

On that note, what are turkeys' favourite plants, bugs, etc. to eat?

I would start with giving them access to as much pasture as you can, once they are large enough to fend for themselves. Carrying capacity will depend on how fertile your site is and how abundantly it is producing things of interest to turkeys. Insects, worms, and other small live things are their favorite food, and they will search over a wide range looking for these. So whatever you can contrive to encourage, attract, trap, or produce more of these will be beneficial. Worm bin? Black soldier flies? Some of the happiest turkeys I ever raised were started off on maggots from a roadkill deer!! Aside from this, they'll eat a bit of most everything else….seeds, greens, sprouts… Fully grown turkeys can swallow, and their gizzards can crack, small acorns, hazelnuts and such like...

Mine really enjoy apples.
As for how much land, it depends but you'll probably want to move them if they've scratch up 30%.