The important things are:
Always draw in Model space and dimension in Paper space
Always draw full-size full-scale in Model space
Always draw using the same units (mm, inches, angstroms, miles, etc) throughout your project
The title block in Paper space should always be drawn the actual paper size (11" x 17" or Arch D, etc). It does not matter if the paper is in inches and the model is in SI.
When you print you print Extents and 1:1.
In Paper space use viewports and set the Viewport scales and UCS to make the object appear rotated and reasonably sized to fit on the paper.
Use dimension styles to present the data in the unit system your client needs it in
When you add dimension in paper space AutoCad reports the object's true size and understands that the object in the Viewport may be much smaller or larger than the paper it is printed on and gets it right.
For example: in Model space 1 unit = 1mm
If you are sending drawings to a company that works in metric, create a dimension style that uses Decimal with Dimscale =1 and "MM" as a suffix.
If your customer wants the dimensions in meters, use a dimscale of 1/1000 and "M" as the suffix
If your customer wants the dimensions in inches, use Architectural and 1/25.4 for the dimscale
If your customer wants both SI and Imperial units, enable the Alternate units with the appropriate scale factors and suffixes.
Under no curcumstances override the dimension text, ever.
There are templates that you can use or create that have the units and styles (dimension, leader, text, etc) that will do this for you, but if you know what you want or need you can easioy setup your own.