I take it then that you'll be recording the stereo output separately to the voice over?
If that's possible, ideally I want to be able to record an external sound source and then lay a commentary over the top of this.
Keeping in mind that the two audio interfaces mentioned in the opening post each have two inputs channels. Stereo sound, comprising left and right channels, will take up both of the two channels leaving no room for your mic.
Would that be an issue if I'm looking to record the mic input separately from the stereo sound?
Another thing to beware, is that with Audacity there's no easy way to record from two audio devices simultaneously, it can however record from multiple channels provided it's from a single audio device.
Hopefully that shouldn't be a problem if I'm recording the two inputs separately
All things considered, especially in keeping to a budget, I would suggest hooking up the AV amp to your existing audio device's line input and buying a USB microphone.
I'm not too familiar with the USB mics available, but I used to peddle Blue USB desktop microphones and I was quite impressed with the performance especially for the price, they're dead easy to set up for a good sound but I have to admit I'm not a fan of the aesthetics.
I've had a Samson USB Go Mic for some time to record lectures and lab notes, with the right positioning it can be made to sound like a much more expensive mic, goes to show that an expensive setup isn't always necessary.
I'll have a look at this as it sounds like a much cheaper/simpler option.
EDIT:
Another option to consider is ditching direct to computer recording for a stand-alone portable voice recorder. I used to do band recording and stuff, which I rarely do nowadays but when ever I need to record something, be it a jam session, lecture or lab notes I resort to my Tascam DR-05 or Zoom H4n portable recorders, partly because I was fed up with the faff of setting up mics and the computer for recording. Still sounds as good though.
I'll have a google about the stand-alone recorders, I have access to a Zoom H1 and Rode Smartlav+ through the marketing team at work so might trial that to see what it performs like