I would suggest that you keep the original copy as a "wav" file. You can always crunch it down to an MP3 later if you wish.
There is a free downloadable programme called "Audacity" with which you can do all sorts of editing. You can export your final files as ".wav" and with a downloadable add on you can also do MP3's.
I'm not sure what you need to link pictures and sound files together, although I have done it to make a slide show with music in the background, I don't remember which programme I used.
Lastly, although this is only speech and not particularly demanding, go for the best equipment you can afford. You will in all probability never get another chance to do it better at a later date.
JohnB