as far as not letting them make changes, look into saving as an e-drawings file.
for adding company info - we add a small extrusion to a handy face with our company name. And we only send out dumb solids - step files usually.
I've also been looking into different ways of doing something similar.
Much of it depends on what type of files they need on the other side. Parasolids are great because it doesn't contain the feature tree with all the parametrics, and is not version specific. The downside is that it strips all the special parts out that you could do in a SolidWorks file, and you could still modify it in certain ways. Extruded text is not very practical in many cases.
You can add an uncompressed Tif picture to your model and this will show up on the screen and many people don't know about it so they can't figure out how to hide or delete it. It's under Insert/Picture, but then to modify it you need to go under View/Display to turn it on/off, or to View/Modify/Picture to Delete or Replace it. It's not a good solution but has potential. It's issues are that you can delete it, or if you use a background image in your system options, that overrides it, and it's screen resolution dependant.
What we need is a logo you can imbed in the template, and be locked in some manner, similar to a TV station's logo on the screen. Doubtful it will ever happen unless a bunch of users request it though, so start requesting!!
Best thing for now I've come up with is to add any text you need to your email as a signature and hope the person you send it to abides, and use NDAs with anyone you do business with and exchange files.