100A sub panel from 150A Service at Body shop
First, don't be pennywise about the size of the panel.  You want a lot of spaces.  Spaces go a lot faster than you think, especially with 240V equipment.  I like to finish an installation with more than 50% of the breaker spaces unused.   I mention that because 100A panels are usually quite small.  

#1 aluminum will suffice, 100' is just shy of the 115' where you start thinking about a size bump for distance.  But if you're considering that... 

One option is to go clear up to 150A, or 3/0 aluminum.  Why would you do that?  It makes the subpanel feed breaker in the main panel unnecessary.  The existing 150A breaker protects the wire and the 150A+ rated subpanel.  So you can just use thru-lugs or a subfeed lug kit  (a breaker that's just lugs). 

Generally commercial work requires conduit - if you use metal conduit, that serves as a ground wire, and you don't need one.