Could/will evolution eventually lead to a single "perfect" self sustaining species

It won't. Evolution has no objective other than the survival of the genes carried in their respective organisms.

The concept that evolution is in some way aiming at a goal, whether it's a super-human or a "perfect" species is only held by people who don't understand evolution.

If you define "perfect" as "longest surviving without needing to change or adapt" then I suggest that Blue-Green Algae are a good contender - they have existed for about 3 billion years and are "self-sustaining" in that they conduct their own photosynthesis

I don't think there will ever be one single perfect species. Evolution perfects traits within species. Eventually all species will reach their evolutionary potential, meaning it can't get any better. I think when this happens it may start to go downwards again by mutations or food shortages etc. After this i guess evolution would occur again and the process would just keep continuing.
That's just my thoughts though, as you said the question is pretty unanswerable.
=)

I don't think so cause the environment is constantly changeing so we would have to just keep on changing with the environment