Regarding;
Country you live in it would help me consider options if you could tell me what country your reside in. for my listing of your system I will select Germany, let me know if this is not correct.
Ram your G-Skill Aegis DDR4 3000 Mhz 8GB cost 91 Euro @ best from what I see and that is a 1x8GB (single channel not a dual channel chip)
which will mean you will have to get 2x8GB kit (dual channel to upgrade to 16GB) meaning you will have to buy it all over again. which I find a waste of money as putting 2x8GB single sided ram will only do nothing more then 16GB single sided not dual sided. (not the same chipset on a ram stick)
I would get instead of wasting money: G.Skill Value DDR4-2666 2x8GB 16GB €174.95
https://be.pcpartpicker.com/product/vzkj4D/gskill-value-16gb-2-x-8gb-ddr4-2666-memory-f4-2666c19d-16gnt
SSD I understand that 500GB is more needed than 1500 MB/S (M.2) versus 500 MB/S (SATA) performance, but maybe later your next SSD will be M.2 drive. but chop spending on the case and you could get a nice fast Samsung MZ-V7E500BW 970 Evo M.2-2280 SSD 500GB for €175.84 (ref https://de.pcpartpicker.com/product/P4ZFf7/samsung-970-evo-500gb-m2-2280-solid-state-drive-mz-v7e500bw)
Video Card you failed to mention what video card you intended to use with this new system
Case personally I believe if you can spend over 100 Euros for a case you need to get a 75 euro case and get better internal components. but to each their own.
Power supply your system runs about 200W (without a video card) so I will take a guess a 750 will be a bit overkill, you likely even with a GTX1070 video card run a 550W without an issue. (I suggest one below listing) and save from a 140 Euros 750 and use it to better the internal of your system.
all this said I suggest revisiting your system in this fashion, you can compare it to the price you are looking to pay, again price may change once you tekll me what country your in.
PCPartPicker part list /
Price breakdown by merchant
CPU: Intel - Core i5-8600K 3.6GHz 6-Core Processor (€238.69 @ Alza)
CPU Cooler: Deepcool - GAMMAXX GT 29.5 CFM CPU Cooler (€50.62 @ Amazon Deutschland)
Motherboard: Asus - Prime Z370-A ATX LGA1151 Motherboard (€155.82 @ Amazon Deutschland)
Memory: Crucial - Ballistix Sport LT 16GB (2 x 8GB) DDR4-2666 Memory (€159.71 @ Amazon Deutschland)
Storage: Samsung - 970 Evo 500GB M.2-2280 Solid State Drive (€175.84 @ Amazon Deutschland)
Case: NZXT - S340 (Black) ATX Mid Tower Case (€74.69 @ Alza)
Power Supply: SeaSonic - FOCUS Plus Gold 550W 80+ Gold Certified Fully-Modular ATX Power Supply (€80.89 @ Alternate)
Total: €936.26
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-06-06 14:42 CEST+0200