Appreciate the tips here.
I just bought a cheap Nokia Lumia 610 with Windows 7. It turns out that in addition to 'who/what/when/where/why/how' situations, it will also add a question mark if I start the text with 'Do you...' (as in 'Do you want to go for coffee at 10:30?').
This is a great improvement over my initial efforts (i.e. 'Want to go for coffee at 10:30 question mark.') !
Also works with 'Can I...', 'Are you...', but not with 'Shall' or 'May'.
My phone doesn't seem to have the 'add more' feature, but it does add a period at the end of the text. I guess I'll have to manually add periods if I'm texting several sentences.