The 1262 has dual 4 ohm coils, so each driver can be wired for either 2 ohm or 8 ohm. That allows you to wire a pair of the 1262's to 4 ohms, which works best for most non car audio equipment.
The 1260 is a single 4 ohm coil, so these drivers are ideal for individual use.
The NU3000 will provide the right amount of power to maximize a pair of the infinities per channel. If you go with a ported box you need the DSP version of the amp in order to set a 20 hz high pass filter, if you go sealed you can get away without a filter, but that also removes the possibility of EQing your sub.
The ports can exit where ever you need them, as long as the inner opening is 4" form the inner wall, and the outer opening will not be pushed up against a wall. You can modify the height of the sub to allow for the ports to down fire, and make the legs 4" high. ABS pipe is very strong, 1/8" thick plastic, connecting it to the cabinet at two points is really all that's needed.