There seem to be a gazillion open-source or quasi open-source ESB projects. Like so many open source projects, Mule seems to have spawned a commercial version which adds features and support.
I would certainly take a look at
the Apache Software Foundation's version.
If this was my problem I would start with a list of all the industry standards which the various parts you need to stitch together are using.
There are also many related technology directions with lots of fun acronyms like BPEL. I just attempted
this BPEL survey article. (sorry about the membership thing but it is painless)
Bill