Which Linux distro best to install on a Mac mini 2011 server?
From a few minutes of searching, it looks like Ubuntu 11.04 would be a good fit and match most of your requests, and what you were looking to use as well.

64bit would be needed to reach more than 4GB of RAM. And there is a 64 bit version, although (to me) confusingly named AMD64. This would be the version you need. 

There are also Magic Trackpad drivers available, and a good writeup available on the Ubuntu Wiki.

I also found a good site documenting installing 11.04 on a MacBook Pro. This should be rather similar to your instal. 

Finally, in most cases, choosing the right Linux distro is also a feel and how you like the install. But in accordance with your list of support, it looks like Ubuntu will cover this.