As extensive 727-100 user i say:

Michael2 mod for flight model required if you want to fly by the book. its realistic, but adds some ground time to plan, calculate flight.

sound fix definitely required, and you can choose which one. with 707 gau file as CoolP said, or some editing + doule loading aircraft.

Hex editing fixes from Virtual Captain required IF you want to fix minor bugs, which not affect flight. (like A\P HDG hold button in VC, Oil quantity test button)

Pressuration fix from Dutch definitely required.

Stab Trim requires some care. you may want to decrease sensitivity in aircraft.cfg or just disable click repeat in FSX controls tab. If you use Michael2 mod - dont edit aircraft.cfg or you wont be able to set takeoff trim by table.

Thats about all. You fix it, and it'll fly by boeing manuals 8-)
Michael2 mod optional - CS default model flies fine, but for real super true absolutely realism - you gotta try it ;)

P.S. You might want to edit some textures to add some custom scratches like Mike loves Marry, or photographs of girls or this kinda foolish things ;) just kidding :)