Dean's much more knowledgeable in this sort of biking-field then I am. And his suggestions seem sound for the riding you're aiming for. Definitely take into account what he's said!
In regards to your latter question about what a stem and spacer are:
The Stem is the component on a bike which connects the handlebars to the steerer tube of the bike's fork. Seen
here and
here are the two different types; quill and threadless respectively.
Quill stems are, almost always, used for bikes with no suspension. Rigid forks suit the quill as it actually slots into the steerer tube. Whereas a threadless stem clamps around the steerer tube and is used more with suspension-laden bikes.
Its not a general rule of thumb, but it does count with most bikes.
The spacers I'm going to let Dean explain. I'm unsure what spacer type he's talking about. Plus, he'll know more about them regardless of the location on a bike such as a hybrid, road and general here-to-there bike.
I'm 6'4, and I go for Large frames, but then again, there are differences in sizing between mountain bikes, road bikes and downhill bikes. Check
this out, though, I'm sure you'll find the size of frame you're looking for with this.