You don't necessarily have to. I haven't finished it myself (and I have basically no recollection of the few episodes I watched years ago), yet it doesn't really lessen the impact of watching EVOL - I guess there are a few robot moves carrying on, some of the music (even if you haven't watched Sousei no Aquarion, though, you've probably heard its OP somewhere a few times :P), and above all, the fact that the characters are all reincarnations of the ones in Sousei no Aquarion, buuut we don't even know who's who yet, and I'm pretty sure it'll properly be explained if it's relevant to the plot -- it's not like the original series was very popular to begin with. So yeah, just a few details that act as nice fanservice for the previous fans, EVOL works great as a stand alone show.
Avatar is Kagura and signature is Cayenne, two characters from Aquarion EVOL. The pictures are taken straight out of the ED illustrated by the original character designer, Kurahana Chinatsu (the woman behind most of the Nitro+ Chiral game designs).
I feel like I'm trying to drive everyone into watching it these days, but nevertheless I'll repeat it again (that's what being a fanboy means!) - the show's really pretty and overall lots of fun, if you feel like giving it a try, I highly recommend it. :P
I never liked the Saint Seiya manga much. The anime just does everything better. And the manga's art is kind of, well, terrible. But since only the manga was released in English, it's not like you have a choice anyway.
The Isaac episodes were great. More Hyoga backstory is always good.
Prepare to be blown away by the first set of Hades OVAs too (I remembered the first time I saw them like it was yesterday, I was in complete awe *_*). And then be prepared to be sorely disappointed by the rest