How is your neighborhood? You had stated that your home wasnt the worst house on the block which is good, but you wont necessarily want the best home on the block either as the "hood" will be a major determining factor in the price you could get for your home.
If the old vent emptied into the attic, it probably vented grease, soot, and moisture into that space. Your wood probably looks darker especially in the area of the vent, but may not necessarily be bad, just a different color.
That said, how old is your home and roof? The plywood is probably original install which isnt necessarily bad. If it isnt leaking, I would concentrate on the kitchen and baths first and keep your heads up for possible water in your attic in the future. It looks like you have access to the attic, you could easily vent your stove hood through a wall in the side of your attic if available. It doesnt have to vent through the roof, just leave some space between the vent and the eaves/roof overhang if you vent through a wall. In my opinion, a stove hood that simply vents back into the living space of a home isnt desirable.
When you do decide to do the roof, ask around and get a hold of a roofer you know/trust. You can lay a second layer of asphalt shingle easily enough directly over the old ones as mentioned in another post. It may not last as long overall, but will be very functional and look better if thats what your going for. If your tearing off the old shingle, a good roofer/friend will really get a good view of whats underneath and be able to tell you which sheets of plywood are good and which sheets need replaced. You dont necessarily have to replace all of the plywood.
Is the roof rippled? Is it uneven or warped? The wood might be bad in those spots. Are the shingles curled or falling apart? If not, they are okay. If they are beginning to curl, but there is no water inside, they are okay for a season or two but need watching. If they are definetly curled and falling apart, they need attention. In general asphalt shingles should last 20-30 years if installed properly. Sometimes they will have lichens growing on them or appear stained and that is a cosmetic thing, not a functional thing necessarily. It is normal to have exposed nails in the inside of the attic, rest assured there are nails in the trusses as well holding it all together. If the inside of the plywood is uniformily colored, its probably okay. Areas that have water will look like it with circular stains of darker color than the rest of the wood.
If you havent had the home thouroughly inspected as mentioned previously, you should. Again ask around and get someone who will take you around the home with a checklist/plan and spend about 3-4 hours with you and go ever every detail on the house from roof and attic to electrical/heat/cool/plumbing/insulation/interior/doors/windows/floor/foundation/crawlspace/basement. Should cost about $300 - $350 and really will be worth it. You should also get a detailed final written report with pictures and reccomendations about what needs to be addressed now and what can wait.
Good luck
