Fortnite enough for the game to run smoothly?
Rather no. If you only have a budget of just 300 euro available, I would almost advise you to a console. Not only does Fortnite run smoothly then.
Or you take the suggested PC of "Phillip1872r":
Fortnite would be better than the one you linked to PC.
The assembly is really not difficult. ^^
Liquid runs on it no rudimentary modern game.
For reasonable gaming you should have about 500 euro. (Example: https://hardwarerat.de/...arerat-500
Well, probably not exactly liquid. You'll need to turn the graphics down to the minimum and you'll only get 30FPS or so (guess). But for a complete system around 300 euro you can't expect much.
In general, I advise to self-built, because you so much less pay and also have other benefits. And there's actually no excuse, that creates even a layman with the right guidance and some research.
Just send me a message if you have questions.
If you do not want to build yourself, then you can look at hardwarerat.de. There are quite inexpensive complete systems.
But I can only say again: Build yourself a computer! It's fun and you can learn something here.
I hope this helps.
BG
Thomas
(your computer guru)