Well, the data on this website is better than walletinvestors.com mis and over-calculations.
is it possible to buy minexcoin using credit card? or we need to buy bitcoin first?
Good option that you should use will depend on your country. If you are located in the US, you can use coinbase. If not, Try Cex, Coinmama or you could use LocalEthereum instead. There's also a marketplace here in bitcointalk. Always pick the trusted one and remember to have your transaction ESCROWED.