Best audio codec to use in mpeg-2 editing

[walmher]

I´m totally newbie so this questions may seem a little fool.
I have a lot of VHSs that I´ll save in my hard drive as MPEG-2,
don´t think to make DVDs. Some of them will be edited and added
transitions, titles and some video effects but the audio will
remain unmodified. With others I´ll do the same but I´ll work
with the audio too, deleting it in some cases, changing it by
other music or narration background in others, and making noise
reduction in others more. So here are the questions:

1) In the cases where there will not be audio modification,
which audio codec is the best to use in order for intense
video editing doesn´t affect audio quality: LPCM, Dolby
Digital or MPEG-1 LayerII?

2) And at the other side, with audio editing, the same question:
which audio codec is the best?

Thanks in advance.

[edDV]

If quality is the issue, edit PCM (no recompression).

NTSC DVD standard should be AC-3 or PCM although MP2 will play in most players (not standard).

[walmher]

I forgot to say that here we have PAL-NC, however some VHSs I have are in NTSC so your answer is very useful.

Thank you very much, edDV.