One snag to this scenario
The problem with buying one big drive and splitting it 50/50 for storage and backup is - if the drive fails, you lose your stuff and the backup of your stuff.
If you are going to get a storage system to use dual purpose, get something that can protect you from a Drive failure - like a Drobo or something. Drobo will let you do a partition for TM, and allow you to use the remaining capacity (and new capacity you can easily at to Drobo) for your personal storage.
Just don't be surprised if you lose anything if your single drive fails =(
- Eddie