I have convinced my boss that we need a megger, now to find the right one.

We have a lot of motor circuits, and I think that will be the most tested item.
Any suggestions?

We have about Ten (10) -  AVO BM 223 - (s) here at the plant.  Relatively inexpensive and easy to use.

How much is the boss willing to spend?

MDShunk and Brianjohn are both knowledgeable about meggers and might be able to recommend something based on cost.

What size motors, generally?  Normal operating voltage, mainly, I mean.  Do do work in mills and such, for instance, that run their motors on "primary"?

The info requested.
Motors 480 volt, 5 hp and less.
I work in a warehouse/office area.
Since I have never used a megger, I'm not sure what to look for.

Guessing from your post, there is no specific Megger that you or your boss is expecting to use. Since this is your first megger, I would suggest a couple of things.

Google the term Megger... you will find many links that will take you to great sites for info on the megger units themselves and for great info on how to use the megger and read the results. The site for Megger tools is probably the best for info, as you can download "A Stitch in Time".   It is a classic and still has good info. The site also has other good info on meggers. So does Fluke.
After that, you will be able to make a more educated purchase for you and your boss.