"Why would he need to use specialist software to securely wipe the file from the computer, surely just erasing the file in the usual way would be enough for his sister not to be able to retrieve the file, but perhaps not the police? "
We can't really answer that without knowinge exactly what the 'specialist software' is and what exactly it does.
"Do you believe he was just being curious or was there a more serious intent? "
I don't know. But I don't thinking downloading AQ literature is particularly strong evidence of 'serious intent'. On my computer (and until recently on my e-reader) I have a copy of Mein Kampf, and various white supremacist websites would turn up in my browsing history if you looked hard enough. I also own numerous books on serial killers and mass murderers in both electronic and paperback form. You could quite easily put those things into a newspaper article and portray me as an extremely dangerous person - but none of them (as I hope is evident to regular users of this site) is any worthwhile evidence as to my political views, my attitude to other human beings or my intentions. If we're worried about terrorist intent, there are much more reliable forms of evidence for it.