I am an experienced Linux administrator.
I can authenticate myself so you could be sure that I will do no harm to your server / services nor abuse your data. Also, I am ready to sign NDA if needed. I can be verified for example by VAT EU number.
Of course I could write a backup script from scratch but I would strongly encourage you to use already available software and 'just' write appropriate configuration files / scripts to customize its behavior. I am using Debian within my infrastructure, and for my purposes I am usually using backupninja software, which could be widely customized (have many plugins for mysql, rsync, rdiff, ftp, dupilicty, etc. etc.)
If you have large server and many sites, then backup you suggested might take a considerably long time to complete and take huge amounts of space after some time.