electricblue92, Welcome to the forum.
"I was wondering what the first things I should do for a used bike, such as taking it to a dealer, checking fluids, recalls... any advice?"
Go to
http://www.triumph.co.uk/usa/4799.aspx and find out if there are any recalls or bulletin work pending.
Near or over 12K miles? Time for a valve check.
"As far as driving motorcycles (versus cars) is it bad to ride in the 5000+ RPM range for very long."
Won't hurt a bit.
"Also, the bike has a power commander; anyone have any experience with one and have some advice about it?"
Widely used method to alter the tune in order to compensate for aftermarket cans and fine tune engine characteristics.
As you sound quite new to motorcycling I suggest visiting a Triumph dealer for the technical parts. Many m/c shops offer class' periodically to familiarize new riders with the basic maintenance issues...usually they're free. If you haven't had any experiance or instruction I suggest some riding courses.
Brad