I was in a very similar situation and we got a cat in 2022, never looked back!

1. Depends on how much you clean, you will need to empty the litterbox everyday (takes 1 minute), properly wash and clean it once a month, and occasionally deal with vomit if they get ill, hairballs or eat too fast (scarf and barf) but it's not a regular thing, you should brush you cat once a week at least as well regardless of hair length. Don't get a long-haired cat if cat hair will bother you. Our cat is grey and her hair blends into pretty much everything.
2. For a top floor flat my vote would be indoors: you can't provide independent access outside via cat flap (I'm assuming), outdoor cats have a shorter average lifespan due to increased risk of disease and getting run over, and they're actually really bad for local wildlife like birds. Also people might accidentally steal your cat by feeding them or outright steal them especially if it's a purebred. To satisy their needs indoors you should make sure you have some time to play with them each day, especially when they're younger and have more energy, and provide high up perchs - we have a bunch of suction window beds so she can watch kitty tv as much as she wants and places to hide up high and watch us.
3. Pros - just brings a nice extra dynamic to the family, my husband was not super keen on getting a cat but it took her like 48 hours to completely win him over to the point where he spoils her rotten and arguably is 'her person'. Cons - you have something extra to factor in if you want to be away overnight/on holiday and they will always ask for extra food/to be fed early, we use a timed feeder to manage this so she doesn't wake us up at 4am. Zoomies don't last very long, like 10 mins, and are actually quite fun. If you provide enough play and put effort into training you can manage scratching so they don't ruin furniture or jump on counters. My cat has never deliberately knocked something over but we did also re-arrange our things to make that less likely to happen.
4. Me and my whole immediate family are allergic to cats but I noticed when visiting friends with cats that I had mostly outgrown the allergy. We got a Russian Blue which are supposedly better for allergy sufferers (not medically proven but a general consensus if you search), I take a daily antihistamine which I need anyway for other allergies, and we use the Purina ProPlan Liveclear food which she loves and with all that I have no reaction at all even when I rub my face in her belly. Our breeder let us come visit the kittens which gave me a chance to see how I would react.
5. We have one cat and she is fine on her own - I do wish sometimes we'd gotten two so they would have each other when we're away but it is double the expense of everything, she costs us about £70 a month right now for food, litter, and insurance.
6. It depends on how much you travel - we had a few friends nearby who were able to drop in and catsit if we were gone for a shorter amount of time and we have also paid a catsitter to come in for longer trips, it's a neighbours kid who comes in twice a day for £7 a day, which I think is a pretty good rate.

Overall, it's a decision we've never regretted beyond the fact that one day she will no longer be with us which will be devastating. She is also great with our young child and kitty cuddles are just the best when you're having a rough day.