First, I'd have the attic inspected to see if it can carry the weight of what you're going to build up there. Depending on the age and construction of the house, the floor may not carry the weight. IN addition (not to coin a pun), you need to see if the HVAC and electrical systems can handle the additional loads. Both systems are usually 'sized' to the home when it's built. The odd, though, are in your favor that the electrical system may not have to be upgraded. But I can't say from here because it depends on the age of the house.
I'm not trying to rain on your parade but you didn't give this type of info. Now, ... Regarding your question - It would be best to level the house BEFORE you do any construction. (WHy you ask???) Because it wouldn't be cost-effective or practical to do damage to the house when it can be prevented in the first place. Consider hiring an Architect or a Professional Contractor to review what I mentioned. It's money well spent. DO be sure to have your rafter cavities vented at both ends w/soffit vents or something like Raft-R-Mates to keep the under-roof cools. It's the most common problem 'job' that's not done when finishing an attic.