Making My Own Config

Hey, so I'm going to bought a new computer and I'm trying to do my own config for the first time, I've read some articles online and I come up with the config you'll see below, I just want to here your advices because I'm a newby for this stuff:

-Processor: Intel Core I5 8400 (2.8 GHz); I'm still not sure if I should take an I3, but with an I5 I'll not have to change it in a long time
-Mother Board: ASRock Z370 Pro4; Seems like an average one, not too overkill with possible extensions and all. If you know similar one but chipest let me know ;)
-Memory: At this point I'm still not sure if I should take 8 or 16 gigas, I'm afraid 8 will not be enough but I'm wondering if only taking one of 8 now will be a good idea, I've read that it's better to have them by 2 (like 2x4 or 2x8). Not mentioning that prices are incredibly high for memories currently.
-SSD: Intel SSD 545s Series, 256 Go, SATA III; I want 256 Go of SSD, if you know a cheapest let me know .
-Box (I actually don't know the word for that piece on English): Phanteks Eclipse P300 Tempered Glass, Noir; Seems like a decent, every pieces can fit in. Tell me if you know a better one.
-Alimentation: EVGA 600W, 600W; Pretty good one, please tell me if you think there's a better option.
-Screen: Asus VP228HE; I already have a little screen for a second screen, so this will be my main. On my shitty pc I have only one screen of 24" but since I'll have two screens I don't think 21" on this one will be too disturbing. As always let me know if you better ones .

Now you're wondering where's the graphic card, but I already have a GTX 1050 TI wich is 7 months old from my shitty pc, it will be enough for a while.

Thank you for reading and for the future help, I will edit the threads if I decide to change any pieces.