Sounds like everyone's covered it pretty well. A couple of final items: Use ellipse/Isocircle for "holes"; avoid "offset"; construct tangents on ellipses;after setting "isoplanes" use "ortho" for basic box construction.
One last thing; if you do a lot of...
Don't "scale" it!! Just go to "units" and set to your desired system. Also, go to Dim Style and set dimensions to your required system. As long as the original drawing creator kept all geometry @ 1:1 life will be good. If not, then "scale-away"!
