By far the most common food in medieval Europe was bread. This could be wheat, barley, rye, oats, or a combination, but some kind of bread was a staple for nearly everyone. Porridge of some sort was also common, and you could think of this as very much like bread in that it was a grain-based main starch. Porridge is really just a very wet dough that is boiled instead of being baked.

Aside from bread, people would have had a wide variety of locally seasonal plant foods, like vegetables, fruits, herbs, mushrooms, nuts, and so on. In addition, nearly everyone would have supplemented their bread with meat and/or dairy products were possible, but these varied widely. They could be fish in coastal regions or larger inland towns, game where hunting was possible, especially in frontier zones, or sheep or goats in pasturelands, or beef and pork in more intensively cultivated lands.

As far as your last question, it's not really that important for medieval Europe because a great deal of food production and consumption happened outside the market. For example, many peasants cultivated their lord's land as well as their own, and either paid labor or in kind to their lord; they might have gotten a share of the crop to sustain themselves, or a portion of the land, but in neither case did they produce goods primarily for sale.

About the only people who would regularly have purchased a substantial portion of their food before the eighteenth century were city and town-dwellers, who would very likely have bought bread, butchered meat, and other staples. But, what they could afford is not quite the right way of thinking of it, because the medieval food system was very much a "moral" economy (see E. P. Thompsons' brilliant article on that term): it existed to distribute food as necessary to maintain the stability and moral code of the community, not to produce profit. As such, bread prices were tied to grain prices, and the trades governing nearly all food products were heavily regulated. These regulations sought to create as direct as possible a relationship between food's production and consumption, and anyone dealing with the purchase and sale of food was morally suspect at best and outright criminal at worst.