Its better to buy Bitcoin first then go for other altcoins. This way you will be having less hassle and can calculate the total in and outs of your trading. Always target to increase the total wealth in Bitcoin and you can trade on any exchange if you do like this.
Yeah it is great idea. if you have bitcoin already then selling your coins is no problem, but you didnt tell which coins do you want to sell. If you have bitcoin for sale then you do not need any particular exchange. You can sell it anywhere you want. But if you want to sell in these days, I suggest holding because the price is low and you may lose your money.