Buying in exchange for BTC is definitely one of the options. But buying with fiat is not that popular. That said, for some of these altcoins, it is really profitable to do mining. The ROI varies from coin to coin. Need to make detailed analysis, before jumping in to the mining scene.
yes if you want to buy any altcoin you need to pay transaction fee, no matter either you are using any exchange you need to pay tax on it. to me do not think that there is such a big difference between the transaction fee of all the exchanges, so you must have to pay transaction fee i you want to buy any altcoin.