as to drawing existing conditions.
it might be good to start with drawing rectangles of your rooms and overall dims. if you start in one corner of the building with wall tool and work to the other you will have an accumulation of error by the time you get to the other side of the building. then you will face the dilemma of "do i make my walls fatter to make up the difference?"
hence its best to work it out with rectangles first and start with the biggest dims and work to the smallest, this allows you to spread the error out so you can achieve standard wall types.
IF you are also doing more than one floor, it would be best to work out a 2d section first. this will help you iron out errors in the vertical dim. especially stair wells.
i had to do this for a recent job and im glad i did. the stair well went to 3 floors and my plan measurements had small errors in them. if i just did the plan and stacked the layers, the stair well would have been misaligned.
if you want to see a pdf of what i did, just email me.