From there it's probably best if you state a budget as you can go from hundreds of dollars to tens of thousands if you cared to. Do you want to keep your current speakers? Types of music you listen to, and size of room will also help others suggest where to start. How picky/discriminating are you and what have you heard that you've liked and disliked. There are so many variables.
A basic system might be the iTransport, a good DAC (so many to choose from), an integrated amp with enough power to go well with the speakers you already have. Associated cables and wires to connect everything that are not zip-cord, but one of many affordable approaches. Oh, and don't forget to put aside a little extra money for a small jar of colored pebbles to place atop your new components to make them sound even more better :-)
I've had great luck buying on A'gon. There are plenty of good dealers, as well as fine manufacturers who sell direct. There's a certain amount of risk in buying used and having things shipped. The feedback system will give you some basic clues, but there are no guarantees. I know folks who wouldn't consider one way or the other for various reasons. It's really your choice, but A'gon is a great resource for used gear. If you want to experiment and try various approaches to building a system, you can buy used and resell and not lose much in the process if you're savvy to what things are worth.