It is an old article, but take a look at http://atransc.org/selecting-audio-recorder/
With any recorder, make a recording and then listen to the result. Listen to the background sound in the recording ... not what you recorded. If there is a low volume steady rush of sound, the recorder may not be of much use for EVP. Like audio tape hiss, the sound may interfere with the voice.
Look for either a little sound that has occasional interruptions, clicks and pops. The very old Panasonic RR DR60 was a really bad audio recorder because of all of the noise it produced along with the recorded voice. It appears the clicks and pops were used to begin voice formation that was made out of the noise. A recorder that has the noise without the clicks and pops seems not to have what it needs to begin the transform process. That must be added, but best it is added to a quieter recorder.
So the bottom line is that you do not need a really good recorder, but whatever you use, it should not have very much internal noise. You can produce the necessary background sound with a fan, running water, even normal household sounds. You can use your computer microphone if you want, but you should plan on adding sound.
The microphone is only needed to record your voice and to bring noise for voice formation into the record process. A quality microphone is not necessary. All of our study indicates the voice is formed in the analog stage between the microphone and the digital storage stage. Not in the air.
As a technical note, I am beginning to think that EVP is formed by way of a conceptual expression which influences how we assign objectivity in reality. This will make more sense for people who have read Your Immortal Self.
In the model I use, we are the channel for the expression we assign as physical world. In other words, we make the world. Reality is modeled as conceptual space and the physical aspect of reality is objective because we have been taught to see it that way.
In physical terms, think of energy wells. A concept that is well established has a deeper energy well and is more difficult to change. EVP are able to be formed when the energy well is sufficiently shallow to allow our intention to cause a change.
Steady state noise represents a very deep energy well. Steady state noise with frequent perturbations represent shallow a energy well (less momentum), and is therefore more susceptible to our influence.