realistically for 1080p 144hz if you're looking to play fortnite with lower settings for the sake of framerate you could really pick up a system with any of this generation's gpus and be totally fine for that with plenty of room left to be doing nvenc encoding on nvidia cards, unsure how streaming might impact the performance for you with amd cards.

If you're interested in running with higher settings while streaming you'd probably want to get something with a 3060 possibly even 3060ti in it to have enough power to output 144hz and stream with nvenc encoding.

I'm not particularly well versed in current cpu markets particularly since the release of intel 12th gen but the absolute best bang for your buck for quite a while was the 5600x any system with that or i5 11th/12th gen intel should be more than sufficient for what you've described though.

Looking at NZXT's offerings in particular since they're one of the easier places to find your white case you'd be looking at anything from their lowest "Starter PC" prebuilt all the way up to their "Streaming PC" prebuilt being suitable for your performance needs depending how high you wanted to push your fortnite settings with anything above that being overkill. I'm not sure how often they might do sales and mark things down but Black Friday is coming up relatively soon and surely they would have deals at time of year to watch for.

Also if you do find a prebuilt that's particularly appealing to you anywhere that doesn't already have wifi built into the mobo it's a very cheap and easy addition to get a wifi card to slot in yourself.