My experience with digital calipers has been poor. I have a small one I need to return, it holds a zero so poorly, but I needed to do some metric measurements. I plan on getting a metric dial eventually. You can see this when you open and close them repeatedly, and they loose their zero. Even higher end ones do this, I've yet to see one that didn't, but I've limited exposure and probably haven't seen anyone spend enough money for a really reliable one.
STL files are the ones generally used by 3D printers. Also, most CAD programs don't do a whole lost with them without reprocessing them into their native format.
Another thing to note, but in passing, is that Nerf stuff is made overseas, and made to metric specs, 12mm instead of .5 inches, for example. The two don't match exactly, so if you're measuring in inches, you'll get odd numbers. If you try to round them, you'll be off by varying margins. Something to keep in mind, nice thing about Solidworks is it'll convert the two, so you can put a metric into an inch model and get the right dimension.
I've never had any issues with digital calipers of any brand - I've used both Starret and Harbor Freight (essentially the best of the best and the worst of the worst, respectively). Littlemachineshop.com has some nice intermediate models that work quite well and are priced reasonably.
True, but there are still relatively few printers in the community. I have access to a couple very high-end ones through school, as do a couple others, and 3 or 4 people have RepRap variants or Makerbots. Many, many more people have access to CAD, and .stl files import as triangle meshes into SolidWorks, which are a pain to edit or mate to. Most other CAD packages won't even let you open them (Inventor, for example, which is in widespread use because it's free to students). I'm not saying release a STEP instead of a .stl, I'm saying release a STEP in addition to the other formats, so that more people can use your files.
Unit conversions should not be a problem. When you start getting down to 4 places past the decimal, those measurements vary significantly with temperature if you're measuring correctly. A few thousandths is probably better than the tolerances present on the actual molded parts. However, you're not supposed to state a tolerance on the CAD model - it's supposed to be absolute. Drawing sheets state a tolerance because they're inspection tools and are supposed to tell an inspector how far off a part can be from the nominal dimension before it has to be rejected.