I'm with Sketch on this one; I dislike reusing main character voices if I can help it, and I can only really do two young-ish teen type voices: spunky shonen (Van Fanel) and whiny angst (Kei Kurono). I'm afraid that anything I tried to do for an adult Siraba would be caught somewhere floating between the two of those.
Voicing Scar would be an absolute treat, though. He's one of my favorite Disney villains, and certainly my favorite character from Lion King.
There are definitely advantages to the main writer doing the editing, but if you've never done any serious editing before, Lion King might not be the best place to start. Generally speaking, the more visually elaborate and high-quality the animation is, the harder it is to work with, and it doesn't get much more elaborate and high-quality than a big theatrical Disney picture from the 90's. For someone with even a little experience under their belt, that might make for a fun challenge, but for someone who's still struggling to learn the basics, I could see it becoming frustrating.
Don't get me wrong, I definitely suggest learning to edit to anyone who's interested in getting in on the abridging thing. It's an immensely fun and rewarding hobby. But you may be better served by starting with a slightly less arabitious project to get your bearings and coming back to Lion King later-- it'll almost certainly still be there when you feel up to it.
Or if you're able to find a willing editor to do it for you, you could jump straight into it now, but don't look at me. (-: I have enough on my plate this spring with my own projects, and starting this summer I'll be taking grad classes and hence have less free time than ever.