Hi OP! if you want to read something not so opinionated about cats and something related to your actual query, here you go.
We used to live in a 1st floor flat, next to a busy bus route. We got our kitten (now 3 year old cat) as soon as she was able to leave mom, and she was an indoor for the entirety of our time living at the flat. We moved from there now due to personal circs but I can help with the initial questions for you.
Depending on if you get a short or long haired cat, consider their food and grooming situation. If long haired, be ready for daily care.
Outdoor cats naturally eat grass and plants to help with fur balls etc, so indoor will try and source plant like material. Be wary of plants you have in your home which may be toxic to cats (flowers like lillies). You can grow your own cat grass very easily on a sunny windowsill.
Youve said you’re getting a male, while he wouldn’t be going out I would still say it would be essential for him to be snipped. If he ever escaped he could cause chaos, and he may sense females in heat from inside and begin spraying inside and trust me, it’s strong.
He will need microchipping, to be jabbed yearly, and regular flea and worming treatment. Even if he isn’t going out you and your family and visitors are, you bring things in. Always best to keep him protected.
Whilst they’re inside, they’re going to need entertaining, and at the same time space of their own to retreat to. We got cat towers, more toys than we can count, and ensured there were hidey holes for our little one to go to if she got overwhelmed by the kids.
We put food separate from the water, they prefer this naturally, so water was in the living room food in the kitchen. Litter tray is obviously an essential too.
Regarding keeping windows open, for the top windows we were able to use insect mesh cut to size and Velcro’d to fit. As she got older, we upgraded it to metal mesh as she began ripping the material. We only had one incident where she got through the material and walked outside on the windowsill and she didn’t have a clue what to do, we just grabbed her back in.
Most important thing is at the start do not overwhelm the little one, when they get there they’re going to be away from mom and wondering what’s going on. Gentle love, gentle cuddles, allow to explore, Feliway diffuser if absolutely needed, but patience is the key. Don’t give him full roam immediately, keep him in one room to get him calm for a few days. I slept on the sofa with our cat when she first arrived for about a week before letting her wonder the rest of the home.
Good luck! xx