Hello dear,
Getting the stroller onto the first floor is not such a disaster. In theory, you can turn it around and pull the stroller slowly behind you on its rear wheels. This way, you wouldn't need even lift or carry the stroller in any cumbersome way. Anyway, I would certainly pick something relatively lightweight and with excellent driving abilities - that would be the priority. Having a good center of gravity is more important than the weight itself -so I recommend to take the opportunity and visit a shop to try some out. Also, lift it as you are going over a curb, LOADED with ideally at least 8-9 kg. A heavy bag will do; some stores can provide dummies.
Shame you didn't mention your budget. It is a world of difference looking for a stylish, yet functionally designed stroller within an unlimited budget, compared to choosing from more affordable models. At least, I will try to inspire you with some choices. You can always send me another question with more details - your budget, terrain, or other preferences - not just the weight.
In the meantime, check the following strollers under 12 kg: ABC Design Salsa 4, Jané Kawai, Moon Style, and perhaps Stokke Beat.
When it comes to excellent maneuverability, have a look at the Britax Go Big. It is not a lightweight stroller but comes with exceptional drivability. I think with this stroller, you will manage to get it to the first floor (rear wheels first) fine.
On the other hand, the Easywalker Harvey is also a good choice.