Newbie stuff: Best value in cat foods? Dry or canned?

We have sort of "adopted" a cat that visits us once or twice a day. I
wouldn't mind keeping her but she is a free spirit and I think
confinement would be a torture for her now.

Her behavior is quite aristocratic: For example one of her visits is in
the late aftrenoon. She suns on the porch, then waits by the door and
wants to come in as soon as she sees us. She likes to lie on various
carpets, sit in a few of her favorite spots, be petted, watch TV for a
while (she really likes TV!), have some food and then she definitely
wants to leave and move on! She quietly sits by the door once again,
this time to be let out. She likes to spend about 2 hours with us.

She never misbehaves, never pees inside. She never begs for food, never
jumps at food even if she is very hungry and seems to prefer to eat
alone when no one is watching. (Typically, when I place the food, she
checks it out, has a bite or two at most, but then gets busy with other
life. She prefers to eat later when we are at the other end of the
house. Any explanation?)

We know nothing about cat foods. So far I have bought a dry bag and
occasional cans from Target, Walgreens etc. A little bit of web surfing
tells me such foods are likely to be "unhealthy junk foods", with meat
by-produts, grains etc dominating. I have a few basic question for the
experts:

1. Is dry food better or wet food, or some kind of rotation?

2. We can't afford to go too premium but also don't want to feed her
junk food that's bad for her. Are there any "best value" brands to look
for? What kind of stores are best?

3. If she doesn't finish the food, can the left-overs remain in the
bowl to be eaten later, or should they be discarded?

4. In case of canned food, if we don't use the entire can, can the
unused portion be refrigerated and used next day? Should it be given
cold, or microwaved?

5. Do cats like to eat at a definite spot, from a fixed bowl, etc?

As you can see, we know nothing. All pointers would be greatly
appreciated.