Howdy.
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I think you should research Blueprints, check out the Blueprints subforum as well, they are quite powerful and I’m sure you could build a decently complex game with them. I think you’ll find the combination of Blueprints and C++ the most powerful. I’m a software developer by trade (C++ is my actual job) and I’m so far preferring Blueprints. Even if you don’t know the language very well, you don’t really need to. I suggest you start in Blueprints and keep going until you can’t anymore. Your knowledge of programming concepts (which you have if you’re fluent in Lua or any other language) will help you understand all the underlying concepts of Blueprints. You may not even need c++ at all, but in the event something doesn’t work the way you want or you want to expose something that isn’t exposed to Blueprints by default, you will know enough (from knowing like I said, Lua or really any language) to be able to find the answers to get around it in code and expose it back to Blueprints.
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See above. 
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Sort of? This is a complicated question to answer because it depends on a lot of things, so forgive me if I am a ramble a bit here, I apologize in advance.
First it really depends on what ‘decent graphics’ means to you. You mention mirrors edge, which is a nice example. If you went with that same motif for your game, you could probably cut it down to a minimum number of assets. You could spend some time building some really high quality modular buildings (remember mirrors edge is very sparse, there’s not a bunch of desks with papers and pencils, it’s lots of blank space, colorful but otherwise blank, very minimalistic but also very futuristic and sleek looking) and put together procedurally generated cities if you wanted to (through blueprints!). You’d spend a lot of time on the model of the player (since it’s true first person and they see their arms/legs very up close etc.) Even at this, you’re looking at - for a full game - a really impressively small list of things you need. You could definitely pull off something similar within a year if you really put your nose to the grindstone.
Second good gameplay is a whole different can of worms. There’s a big part of this that I’m not even going to try to touch on, it’s much too broad of a subject to be relevant here, and we could have discussions on just this part alone but - we’ll ignore that different games are fun for different people. Even if you made the best game ever, I might not be a fan and might not find the gameplay ‘good’. That is super subjective, and is why there are so many subgenres and niche successful games and why, hell, different genres themselves exist in the first place. So we’ll skip that. Let’s pretend that whatever type of game your making is one that is enjoyable by everyone, we’ll just pretend for a minute that it’s going to be an RPG (for no reason other than I just want to give one example, don’t read anything into it being an RPG). I’m going to assume a little bit here, that you’ve played an RPG before that you liked and can think of it (Fallout? Zelda? Skyrim? Final Fantasy?) When you think about why you like those games, and why you remember them - well the things you’re remembering are what you need your game to evoke in other people. Do you think that your ideas will leave people feeling the way those games did? Do you think you’ll be able to establish a similar connection through story? Or do you think you can create an in depth battle system or other mechanic that will really be enjoyable to people who play it? Because that’s what it boils down to.
If games were a simple matter of just having pretty things, you wouldn’t need any gameplay - you’d just need artists. You could take your amazing assets, import them to UE4 - which is an amazing engine, and can give you the most polished looking beautiful levels/worlds whatever. But if you just plugged that into the default “third person template”, where you can run and jump and move around, well, no one would play it. It wouldn’t be a game. If you did the bare minimum and made it give you a score for the further you ran, it’d be a pretty boring game but it’d still be a game. Here you can see that (if you agree with that example, which I don’t know why you wouldn’t) without good ideas for mechanics that players enjoy, the graphical fidelity doesn’t really matter, you could be a savant and still have a ‘game’ no one likes.
However, if your game is fun to play, it’s no where near as important that it looks amazing. This is how games like Dwarf Fortress thrive, it barely has any ‘looks’ at all (besides fanmade tilesets, which don’t really count.) yet it has huge amazingly active communities, enough that they’ve supported the dev for years on donations alone for a game that’s never left alpha. Why if graphics were more important that gameplay (story, mechanics etc) would people still regularly play games like Fallout 1 and 2? Graphically they are miles behind current games, but they’re good. Why play Mario N64 when graphically our phones are more advanced now? People still play it because it’s a fun game, not because of the way it looks.
TL;DR for 3 - Yes, if your idea of ‘decent’ graphics is reasonable for a one person gig (or you’re willing to pay someone else to create assets for you, you could probably find reasonable prices for talented people here on these forums), and you can handle/come up with the ‘good gameplay’. No matter how amazing the engine is, if your game requires a good story and you don’t have a good story - it can’t help. No matter how great the rendering potential of the engine, the power of all the technology, if you don’t provide it with quality assets - it can’t help. If your game requires an indepth combat system, and you can’t find a way to make it intuitive and fun to play - the engine can’t help you there either. Unreal is a tool, a very powerful one - but you still have to make the game.