There are a bunch of good posts here, but most people are jumping to solution without knowing some pretty important requirements.
1) How important are the backups ? Is your system backup failure rate is 1% is that ok ? How many backups can fail, affects how much effort you need to put into the validation you need to do to make sure that you've got every backup. I'd suggest you should use whatever you use for regular backups, automate the agent installation and removal - this way you can plug into your regular corporate backup system - and avoid creating a whole load of bespoke process.
2) How important is it that you wipe 100% of the disks ? if your wipe failure rate is 1% is that ok ? (as above - if the answer is no, then you need to put the correct amount of effort into the validation that you've wiped the disk, and into creating an audit trail to prove the wiping was done).
3) Are there certain machines you really, really need to have copies of and make sure are wiped ? eg the CXO's ? If you make sure the really important ones are dealt with, can you accept a higher failure rate on the others ?