Looking for Memory - Need help choosing

If I'm getting a MSI Z270 TOMAHAWK, and a G4560 for now, expecting to get either a Coffee Lake i5 or i7 later, which RAM should I go with now? 8GB now, then expect another 8GB later.

I'm buying from Amazon because I have a $50 gift card. Should I buy a cheap 8GB stick now (possibly sell it) and then a good kit of 16GB later? Or a good 8GB stick now, and another good 8GB stick/kit to go with it later?

My options:

Corsair Vengeance LPX 8GB DDR4 DRAM 2400MHz C16 - $58.99 (on sale, originally $66.99)

Corsair Vengeance LPX 8GB (1 x 8GB) DDR4 DRAM 2400MHz (PC4-19200) C14 - $66.99

Corsair Vengeance LPX 8GB (2x4GB) DDR4 DRAM 3000MHz (PC4-24000) C15 - $71.99

and of course take $50 off for gift card. What would you do?