Hello everyone, im a starting game developer and i have few questions:
I try to answer this stuff 
1.) You do not need any language like c++, but it helps. Most the stuff you want, you can build in Blueprint.
2.) If you are new maybe you should start with simple game mechanics. It helps to start easy and become more complex. Explicit to your question. UE4 should be able to handle it. Stuff like view distance is up to you.
3.) This is quiet hard to answer, cause it looks like you don’t know the basics of UE4 (or game development). You should start with the provided tutorials. Stuff like leveling is quiet easy to archieve, talent trees (especially with skills to use) or items are more advanced and will cost you some time.
Overall my personal opinion:
To start with an rpg-open world project without any experience befor is hard (not impossible, depends on your ability to learn and improve). But an open world need a lot of time to make it interesting. It’s not like there is a tree. And 2 more, and 3 more. A world is dynamic and needs a lot of stuff in it. Stuff that moves, that interact with the player. I think it’s advanced stuff. But thats only my opinion
Hope I could help you.
Thank you Jack, it helped alot, my brother learned C++ hope it will help us Dont you know any good resource of RPG developers where rpg elements are shown from inside or something like that?
Actually I build everything on my own (except models, meshes, stuff like this. god save my artist :D).
I am a game designer and game programmer. I do stuff normally on my own. I.e. if you want a level system, your brother can make a new Character class, with some new properties. I.e. float experience. Then add a uproperty int32 for the actual level. Then add a function maybe called addExperience(float exp). In this function you add the exp to your experience. And if your exp is higher than i.e. 100, you add a level, and reset the experience to 0.
The normal approach for developing is to take a big problem like “Inventory-System” and seperate it in many many small problems. Like items. Items u seperate in stuff like name, id, attack damage, defense… If you have created the code (in blueprint or c++) then you can think about the next step. maybe the way to collect it. After this you can think about where to store it (inventory). Then you have to think how you interact with the inventory.
Stuff like this. Hope it helps a little, even without any great link to a source 
Thanks you again for helping such a noob like me :), i have one more question, as you mentioned inventory,items etc - can it be done by blueprints without C++?
Actually cause I do most the stuff in c++, I’m not totally sure about it. But I think Epic made Blueprint pretty strong, so I think yes you can.
I.e. my technical artist started in u4 2 weeks ago and made a Blueprint only demo, where you can flip coins and drop them onto the ground. then you can go there and pick them up. after picking up the hud will tell you if it was a copper, silver or golden coin. then it will count everything together (1 for copper, 2 for silver, 3 for gold) and will tell you how rich you are.
Actually this is basicly all you need to build an item system.
So a complex one should be possible too.
Thats great, thank you again.