First as you're in UK, I should look for a local LUG, Debian is very popular and there should be someone experienced who can help you through things, for a few beers.
Problem 1
http://www.myopenrouter.com/article/...-Quick-Review/ has a link through to a Linux driver.
Whilst i have installed and used Lenny, it'd be hard for me to work through the driver installation; probably the best place is Debian forums, if you can't find a local expert through a LUG.
This process may involve building some kernel driver source, against your kernel header source files, in which case you'll find working through the excellent Debian Documentation on tools required to build a custom kernel helpful.
Problem 2
RAID 1 (mirroring) and LVM are designed for data integrity and easier management of systems with multiple disks. LVM for example allows resizing of partitions.
They are NOT backup solutions. For your need I should mount the external "Backup" space in their own filesystems, and then use the tool rsync(1) to create a shadow copy. "rsync -cSax" creates a checksummed archive copy (time stamps, ownership retained) of a whole directory tree, to another location. On updates only changes are recopied, making this efficient enough to use over a network using an encrypted data stream via "rsync -essh -cSax".
Finally, I think you'll find your "/tmp" is way over sized, unless you have some plan to use it to store large number temporary files like DVD/CD ISO images before burning.