Leveling out a basement floor
I think you should just leave the floor slope as is. It is designed to help support drainage to one end of the basement in the event that there is ever an accumulation of a large amount of water into the basement space.
You should ask yourself if it feels awkward to walk around on the existing floor with its slope. Most likely it is not a big problem at all.

The varying length of the studs should not be a problem because you are going to be cutting them all anyway from the standard lengths that you can buy. They come either as 8' long or as studs they will be 92 5/8" long.

After you have finished out your office it there is a need for anything like a bookshelf or a desk to be perfectly level or plumb all you have to do is to shim under it with wedges or adjust its legs.