Smack that "friend" across the face for starters. They should have told you to go to PC Part Picker to look at price comparisons and see the actual library of PC Parts.
Never go with 1 stick of RAM, you can do this, but it's never recommended due to timings issues that can arise.
With graphics cards they connect to motherboards via PCI slot. So long as you have one, your GPU will work with it, depending on the CPU and motherboard however will determine how well it actually works though.
Parts I'd Recommend
When it comes to 390 motherboards I try to look for a minimum of 8 3.0 USB ports along with 2 M.2 slots. I also look into EKWB, Deepcool, etc. for waterblock and monoblock options. In terms for the motherboard it's fine, it's not something I'd personally consider, but in your price bracket there is really either the option of slower RAM or an M.2 slot.
9400F isn't a bad CPU by any means, it stacks up very well against a 9700K and 8700K and for nearly a third of the price.
The RAM can be changed with something cheaper without a doubt, but really it's not needed and corsair does have a good reputation with RAM, not much need to change.