I'm not the expert, but:

1.  That funky "Thumbhole" behind the pistol grip on the USR and the lack of a flash hider.  I believe the USR comes without the quick release button for the barrel and does come with the "Special Receiver"--i.e. not a permanantly fixed scope like most of the AUG's have (some do have the picatinny rail, but they are fairly expensive).  USR's are usually Grey (there are some other colors and a "dye job" to change the color.

2.  Correct (the quick release button is not illegal to install).  However, if you can find a registered Trigger Pack (machine gun conversion part at about $5000+), you could install it and do whatever you want to the USR at that point.

3.  Not my Forte--maybe someone else can help with this one.

4.  Prices are about right.  If you can have cash in hand, and be ready to jump, you can occasionally find a shooter AUG for $3k.

If you want I'd be willing to scare you up a USR or AUG.

I know of a green AUG in great shape for $3,300.00


email me if interested.