My 2 cents is: Maybe you should get a few more dives than the minimum of 40 required. I say maybe because of course people vary. As long as you are interested in either leading divers or assisting with students then I would go for it. DM course will also increase your ability with the skills (I'm talking about the 20 skills taught in the pool, such as mask clearing, scuba unit removal, etc.--basic skills such as buoyancy should already be in great shape)--you have to do them to "demonstration" quality, which obviously means you will be improving them for yourself as well. Other than that, DM course does little to actually improve your diving.
I will add that for the 400 Swim test, training to get a good time may be in order--this, unlike the other stamina tests, involves good stroke technique and in shape SWIMMING muscles (ei. running to get in good shape is probably of very marginal help).
Liability: This has been discussed in depth and a common theme was that it doesn't help really for a DM or Instructor to just present an AOW card when vacation diving. If an incident occurs, lawyers will probably start by finding out the certification of those involved and perhaps of all that were simply at the scene.