What's the computer system requirements that required to make an excelent graphic realistic game using UnrealEngine, how much does it cost?
Realistic graphics don't just depend on the engine -- the art plays a big part in it. You'll either need to learn to create high quality art, purchase high quality art, or team up with someone else who can create high quality art.
If you're working with Unreal Engine you'll need a computer that meets the minimum (but preferably the recommended) system requirements. If you're planning to also create your own art you would need a machine that meets the requirements for the graphics software as well -- these may be more or less than what Unreal needs. You might for example learn to use the freely available Blender, which will comfortably run even on a weaker computer than you would already require to use Unreal.
I'll leave it to you to price a system, as I don't know where you live, what currency you use, what your local prices are, etc.
What's best programming language used in game development? Is it C# or C++, which one is harder and easier to understand or it's just the same?
There is no "best" language. Both languages have positives and negatives, and a large amount of it just comes down to personal preference. Generally, most people consider C# to be easier than C++. You'll need to use C++ (and/or the Blueprints visual scripting system) if you're working with Unreal, while you would use C# (or UnityScript) if you're working with Unity.
Aside from budget for computer, what else must I spend my money for game development?
You might need to purchase assets (3d models, textures, audio files, etc.) for your game, or to pay someone to create those things. You may need a lawyer's assistance to write up contracts, look over deals from publishers, etc. You may need to spend money on marketing your game. You might need to purchase learning materials.
Instead of creating engine and using one that already existed, is it wise and can raise revenue if I sell it? Should I learn to create my own engine or is it wiser to use someone else's engine?
Which do you really want to do?
If you just want to create one or more games as efficiently as possible and there's a suitable engine available then there's a lot to be said for using an existing engine to save you a lot of time and effort. If you want to create your own engine for the experience or because there isn't an existing engine that suits your game then that path is also perfectly valid. It's really up to you.
I'm not sure what you're asking with the first part of this question, but I think it might be about creating your own engine and then selling it to others? This is certainly possible, but very difficult and time consuming, and unlikely to get you much money given the absolutely fantastic engines that are already available either for free or for some pretty fantastic prices -- I wouldn't embark on the project of creating your own engine with the hopes that you'll be able to make a non-trivial amount of money licencing it to others unless you're creating a very specialised engine that you believe there is demand for and which isn't already available elsewhere.
Hope that helps. :)