welcome to our community :)
As soon as you got your residence permit for 3 years you are allowed to work. That includes working self-employed.
I can't give you a general answer on how expensive the rent is going to be. That depends on too many factors e.g. in which area of Berlin you want to open your shop, how many square meters you want to rent etc.
We have had similar questions to your here on the platform in the past. Maybe the information given there is helpful to you. The first link provides insights about self-employment in Germany in general, while the second one focuses on the topic of opening a restaurant (including an overview of the paperwork that needs to be done):
Furthermore I will link @Marcel who is an experienced lawyer concerning labour market access. If you need detailed information e.g. concerning the required papers he will probably be able to help you with that.
I hope my answer was helpful to you. Feel free to ask further questions if necessary.