Hello there and good luck with your first gardening. It becomes addicting
Here would be my answers to your questions. I didn't read the other answers so I would not be influenced hehe
1 : I prefer to start my plants from seeds because I can enjoy a much bigger variety and because I love to look everyday at my containers and see what germinates, what's harder, how to do better, and more. Also, it's cheaper to buy a $2 seed package which you can often use for 2-3 years. A grown plant is much more expensive.
The disadvantage of starting from seeds would be that it's more work and your seeds may not last more than a couple years. If you have leftovers, they may not germinate after that so you have to verify if they are still good. If you forget, like I always do, you may plant seeds that won't germinate and, when you realize nothing happens, you'll have to get back to the store to buy new seeds and start over. In a situation like that, your plants are late, but is not the end of the world though, I can tell you. I live in a zone 4 and I'm always late on everything because I always forget stuff. I still get a lot of veggies. A couple weeks of good temperature and it's awesome how outdoor plants will grow fast.
On the other side, the advantage I could see for you to buy plants is that they are already big, so you have less to worry about and you may get veggies sooner.
The disadvantages is that it's more expensive and you may have some insects or diseases on them. You don't see them when you buy them but they may appear later because some plant had it in the nursery and passed it to the others.
2 : I always do a little research before I buy a new plant. There are many websites that can help you with your choice when you hesitate. For example, they will ask you the color you like, the season of flowering, the water/sun needs, the lifespan, and more. Also, even if annual flowers don't come back the next year, they drop seeds that may germinate the next year. They may not be at the exact same location than the previous year but it's always fun to see what you get back from last summer
Perennials last longer but they don't last forever (from a few to many years, that really depends on which one you pick). If you buy the plants from seeds/bulbs, indications will be on the package. If you buy the plant, I think they indicate it too buy sometimes there's not a lot of info. Also, I suggest you to really do a little research on the plants you want because some sellers will indicate a zone or a lifespan and it may not be accurate.
If you'd like, I could suggest you a couple plants that are not hard to grow. I have a lot less experience with flowers than I have with the garden and I still managed to have a good success with some varieties. Just let me know if you'd like ideas!!
3 : Different plants are meant for different container sizes. If you buy from seeds, the package will sometimes indicate what container size you need, especially if it's a plant made for container gardening. Otherwise, I am not very good at the repotting topic, so I'll pass this one
I hope I could help you with my answers and cya around!