Adding van liner and new carpet

[NemesiS153]

Looking at buying a van to convert that already has sound deadening and insulation (silent coat and dodo fleece insulation), was planning previously i would also use van liner along with the above. How important is this? Make a big enough difference that I should pull out the existing insulation and start again?

Also, I want to replace the carpet, i'm sure it was fitted with the recommend spray adhesive. Can I just apply new spray and new carpet, or do I need to clean all the existing glue?