First, be sure to encourage your customers to at least use Time Machine or better yet, Carbon Copy Cloner (CCC) to do regular backups. This might sound foolish, but there are a large number of folks out there who never think about backups until it is too late. TM is designed for even the brain dead to be able to do regular backups (hourly by default) as long as they keep the backup drive attached.
Create a USB boot installer drive, you should be able to use an 8 or 16GB stick for this. Plenty of blogs detail how to do this, but you will need a Mac to do this. This will be infinitely faster than Internet recovery. Or, if you are willing to go to the trouble, you can create boot install images on a Mac and install to the new drive from that.
TM and CCC may be able to recover data from a failed Mac drive, you may be able to plug it in to a USB dock with SATA drive support ($10-20 typically). To facilitate cloning, there are some decent USB dual docks that can effortlessly clone disks without having to use a Mac or PC to do the clone (under $50 generally). The latter can work as either an attached external dock, or as a unattached cloning device. If it is still readable, you can install the OS fresh on a new drive and then run the Migration Utility to pull everything off the old drive or TM backup. If they have a CCC image of their drive, you can clone the new drive directly from the CCC image.
In short, a lot of different avenues. But, backups are the surest way to ensure your customers are able to pick up where they left off.