Welcome aboard! I vote for Rock River. Good price, great quality, good customer service and lots of options. Check with your department first to find out which brand is authorized for carry. Some only permit Colt, some permit Colt, Bushmaster, etc., and some don't care what it is as long as you can qualify with it. For a duty rig, this is what I would do:
Upper:
16" lightweight under the handguards M4 profile barrel, 1/7 twist is preferred so you can shoot the heavier 75-77 gr. rounds
flat-top upper receiver
quad rail forend from Daniel Defense or LaRue if cost is no object
TangoDown vertical grip
ARMS 40 or 40L flip up backup sight
Aimpoint M2 or ML2 (EoTech works fine too, but is more annoying if you have astigmatism)
Phantom 5C2 flash hider (it actually reduces, and nearly eliminates muzzle flash)
Lower:
6-position stock
Hogue grip
RRA national match trigger
Other:
Side sling swivels (GG&G is nice)
Get a good 3-point sling. These are very comfy for long wear. I like CQB solutions or SpecterGear
Magpul rail covers
light mount and light if you want it
Hope these suggestions get you on the right track. MJD