Beside a recorder, you really want to invest in a microphone to do your interviews. You can basically use any recorder in the market and buy mics that can work with them. Starting out, you should probably try to purchase a shotgun mic with a shock mount that lets you attach it to your camera. A shotgun is more versatile and you'll probably find more use for it in future. At its price, the Rode VideoMic or VideoMic Pro is hard to beat. It uses a 3.5mm mini jack.
If you are going to be doing a lot of interview where you'll be close to the talent, then you can get a lavalier/lapel (e.g. Giant Squid Audio, Audiotechnica) that will give you a closer and cleaner sound. Do note that most external mics would require your recorder to send plug in power to it. Plug in power is not the same as +48v phantom power. These mics would also use a 3.5mm mini jack. Take note that these microphones have limited cable length (and you should not add extension) so your recorder can't be too far off.
The onboard microphones of the portable recorders are generally not ideal for interviews because of their wide pick-up pattern, and it's stereo placement. Compared to a shotgun mic, what you would get is rather distinct background sounds, and the dialogue recorded off the center of the stereo spectrum.
You've probably done your interview by now so hopefully this bit of information will benefit others in future.
