1. Yes, for hard drive storage space. Number of extracts and their size are factors. I'm guessing that log files and backups will consume additional space depending on how many and how long you keep them.
2. Don't think/treat Tableau Server as a duplicate data source. The extracts are just that, an extraction of ONLY the necessary data for the visualization task at hand (or worksheet that is visible). Do NOT bring in data that you are not using!
3. Get as much RAM and as much processing power (CPU cores) as you can get. Especially RAM. How they perform is anyone's guess because we don't know what visualization they'll have, what calculations they're performing, etc.
I know Yahoo! has used Tableau with success and they deal with terabytes of data -- daily -- so Tableau must be doing something right
The simple answer is: yes you can, and I am not aware of any physical limit on storage apart from your hardware/OS limitations.
However, unless you have a monster-of-a-server, your views will be painfully slow if you just dump all the data into extracts without aggregating it.
I would recommend aggregating and optimising extracts to keep them as small as possible, not because of storage limitations but for views performance.