Yes messiah can retarget motionclips with ease, the compose tab is is one of the most complete motion clip mixer you can find, it’s extremely powerful. have a look at some of the videos made by wegg on
also by Joe Cosman (Animate2) they show exactly the power that compose have.
As for the rendering engine, yes messiah is based on Arnold, and it’s not old, it’s actually new, while it’s not using the latest Arnold, but since it was based on it, allot of adjustments and new features has been added to it since the purchase of the Arnold engine. for GI work it’s pretty fast, along with host of other really fast rendering features. I tried C4D, and found the native engine disappointing. I mostly use XSI as my main tool and Mr for rendering, but recent projects done with Messiah proved to me that messiah is really good. have a look at my vimeo account, there are a few animations done with messiah along with rendering.
All in all i think messiah does fit your needs perfectly, but like any software it does have a certain workflow, it’s not too foreign but specific, so there is some learning curve, other than that, messiah is just awesome.
if you have any more questions, please feel free to ask here or in setuptab.com