As to your Dealer vs. Independant Mechanic question, there are way too many factors involved to give you a black and white answer, IMO.
One factor has to do with the level of honesty and competence that each has.
Your Pontiac Dealer MAY have more experience with Bonneville/3800 specific problems and therefore MAY be your better choice (again, depending upon their level of competence and reputation for honest work).
On the other hand, the labor charges for a typical dealership can be WAY too high for simpler repairs, partially due to standard labor charge books.
You really have to be the one who decides, based on some research or personal experience with the availble choices in your market.
The best bet is to start doing some of it on your own. It makes for a fun hobby...sometimes...
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s