Sounds like you are trying to recreate RAID in software. Wouldn't it make more sense to invest in good quality hardware RAID controllers, that already have this kind of monitoring built-in, along with e-mail and/or SMS alerts?
We've been using AMCC/3Ware 9000-, 9500-, and 9600-series RAID controllers for just this reason, and the onboard e-mail alerts have allowed us to replace 4 drives so far, before they died completely (and still within the manufacturer's warranty period).
These do SMART monitoring along with a bunch of other stuff. If you don't need the RAID features, you can create "Single Disk" arrays that allow you to see/use each drive individually, but with all the onboard cache and monitoring features of the controller (we do this in our ZFS storage servers).