If your going down the route of gpu mining depending on how many cards you want to use your going to need a pc with a decent sized power supply again depending on how many cards and a few pci slots and optionally some pci risers to add in more cards.

www.whattomine.com can give you an idea of how much you are set to earn if you know what cards/hashrate your aiming for.

I am a beginner in the world of mining, I want to start investing and this in mining Eutherium, I want to build my own machine and this by buying Radeon Rx 480, knowing that I have access to free electricity , I would like to know more about the necessary hardware, how many cards should I buy to start getting a good profit? is it feasible to install a water / fan cooling system or to install an air conditioner?
is there a anyway to calculate my gain before I start building ?
Your help is very appreciated.

It looks like a scam, not very reliable