Your dad is a wise man, and your friends don't know what they're talking about. Colt is better than it ever has been. They have chrome lined barrels really dialed in, and they have the best extractor springs. They've just been in the game for so long that they've learned a thing or two over the years.
Um, DD is okay, but I don't see anything to justify the price. They use FN barrels, which are good, but certainly not better than Colt. Their new furniture is also fugly as heck. Basically the only thing that sets them apart from Colt is the handguards, but that doesn't even begin to justify the 1500 dollar price tag. Buy a colt and slap a free floating handguard on it if you're really that into fancy rails.
I honestly don't know how likely it is that your department would let you use your own rifle from home, but that seems to be the exception not the norm. Most cops I know are lucky if their department will even sign off on a new grip or something these days. But having an unmodified LE6920 is going to give you the best chance of getting approval, and then you can try to get aftermarket stuff approved once you hire on. You might even get lucky and hire on with a department that will let you have an SBR, which would be ideal I think, so I wouldn't just go hog wild spending a bunch of money on a fancy gun that you might not even want in the end.
You can't go wrong with an LE6920, though. They're just too cheap and too awesome to turn down. I got one maybe six months ago for around 700, and it came with a carry handle and two mags, which I think are like twenty or thirty apiece. Didn't even need it, just couldn't turn it down and it's still in the plastic waiting for a rainy day. At that price, that leaves you 800 for an Aimpoint and other sundries. I would suggest an Aimpoint PRO and a Surefire Scout to start out with. Those are super solid choices that I think just about any department would approve if they're going to approve anything at all. A KAC drop in rail is also a great choice because it doesn't require modifying the rifle, so again good for department approval prospects. They can be had used for around 200. And as long as we're going in a SOPMOD direction I can't say enough nice things about the Crane stock. It's by far my favorite stock.