The gluten doesn't have a very strong flavour on its own (it basically tastes like unflavoured bread) but it picks up whatever you season it with very well. The recipe I make goes for a typical poultry style seasoning with nooch and veggie broth. The stuffing is very umami with a little brightness from the lemon. It's very yummy.

Here's the recipe I use for the roast:

https://www.theppk.com/2011/11/seitan-roast-stuffed-with-shiitakes-and-leeks/

And I make the gravy from this recipe:

https://www.rabbitandwolves.com/vegan-crispy-black-pepper-cauliflower-steaks/

(Which incidentally is also a very good vegan holiday recipe in its own right!)

You might have trouble finding vital wheat gluten. I'd recommend checking either health food or bulk stores, or just buying it online. Everything else should be pretty easily accessible.

While you can make the recipe with the wash-the-flour method from regular flour, it's already a bit of a challenging recipe, and you have to tweak quite a few things to get the proper wet:dry ratio, so I wouldn't recommend it, especially for your first try.

I normally also do a side of mashed potatoes, and then some other veg like carrots+parsnips, or Brussels sprouts.

If you get a lot of specialty items (like faux cheeses and meats) it won't go far. If you shop like normal but just skip the animal products it'll almost surely be cheaper than regular groceries. You can make your own seitan very easily, there are many YouTube videos showing you how. It's just a simple dough that is washed and kneaded to develop the gluten. You can also make your own tofu, but tofu is dirt cheap unless you get fancy stuff so I recommend just buying it.