Unfortunately, your demands are mutually exclusive in current firmwares.
In a short words:
Download Manager is a property of ASUS, so only official firmware has it.
NTFS r/w - only linux 2.6 (OpenWRT) support this feature, but you will loose WiFi since Broadcom modules don't support newer kernel.
Better printer support is in Oleg.
USB Webcam support is really weak in all firmwares.