MGS V definitive or just the default

The definitive experience costs less than the base game here, hah!

Ground Zeroes is actually the prologue to The Phantom Pain. Buy The Definitive Experience, play Ground Zeroes/The Phantom Pain and then just ignore the online if that way inclined.

I got the defenitive. ..look it's cheaper and you get more

Haven't played MGSV, yet but I'm also thinking whether I should get it during the sale or not.
I have however played Ground Zeroes and while it certainly was meant as an "appetizer" for fans of the series, it does actually add a couple of hours of gametime to the whole experience. I personally think it's well worth the purchase at about ~5Euros as it tells a part of the story leading up to MGSV. As someone who hasn't played any other MGS titles I found the game useful to get a... basic idea of the story and setting. It's a lot of fun, too.
Also regarding G2A, do you intent to pay with bitcoins or their... storemoney? (sorry I don't remember the name) Because they'll charge - I think - about 1Euro extra for the common payment methods like credit card or PayPal at that price. Might be worth pointing out. :)

Definitive Experience. Before Sale the standard version costs $20. Now it costs $30. And then they call it "summer sale" ;-)

just the base game, all those skins are useless.
the extra game might be cheaper bought outside steam or traded.