minimum req for media server?

My 2009 2.26 C2D was borderline capable for my preference. Personally, I wouldn't go any older than that. However there are some that installed Crystal HD in older minis with success.

 

It depends entirely on what sort of streaming you are doing.

If you are transcoding a full HD file in real time your going to need something with a more recent CPU, a Core 2 Duo may not be able to handle that.
If you're streaming without transcoding this shouldn't matter at all, any Mini with Gigabit Ethernet port or an Airport card which supports 802.11N would suffice.

 

For streaming, a NAS drive is fine, so really pretty much any Mini that will run the current version of iTunes smoothly will do just fine.