I have the same problem.  I have the cheapest US dial-up and can only maintain
a connection for 3 hours.  If a mirror is running when the connection drops
httrack would normall purge all the remaining files.  No good on dialup.

I always run with no purge set and wrote a batch file to scan the htmls, etc.
and purge only those files not referenced.  It takes about 10 minutes to scan
a 20,000 file mirror.

The package is free for the asking.