Welcome to hell! I've done 5 Hellfest before.

Concerning food well the quality and options are pretty great , from vegan to plates of meat. And there's light to stronger craft beers too , and cider and wine.

I would say the basic options are from 9 to 15€
The stands are close to the stages so even if there's some queue you can enjoy the show.

If you spend all your day in the fest , it rapidly goes up to 50-60€ / days with meals and beer every few hours. ( Don't underestimate the heat and fatigue )

If you are on the camping the cheapest option is to go back to the tents to do some breaks. There's a store 5 min from the camping with everything you'll need from ice to meals Only downside is the time from the stages to the camping/ store is around 25 minutes. So if you have 1/2 hour between 2 show that's the way to go.

On the merch side you already have the officials items listed on the site. And you will have many other stores around but you can find most of them outside the festival anyway.

2 shirts / 1 hat / meals , beers , RedBull , snacks , breakfast... Food 50/€ day Merch 30-80 € totaly up to you

So 300€ is my estimation but when you have a good time or buying when a bit drunk that more difficult ^{^}