Is the aircraft used by NOAA specially modified?
Except for modifications related to instrumentation, not much. The WP-3D was developed from the Lockheed P-3 Orion, which was  a navalized L-188 Electra.

Orion is a sturdy platform. Used during the Cold War as an antisubmarine patrol aircraft, the plane is built to take punishment.

As the NOAA noted in a Reddit AMA

The WP-3D is built like a tough old pickup truck. It can handle a lot of abuse in the storm environment and come back and fly the next day. This is mainly because of the extremely talented maintenance team at NOAA’s Aircraft Operations Center (AOC)

Simply put, it is a normal aircraft with extensive instrumentation, albeit maintained well. The same applies for the WC-130J used by the 53rd WRS:

The 53rd WRS uses the WC-130J to penetrate tropical storms. These aircraft are not reinforced in any way in fact the only differences between a WC-130J and a C-130J is the addition of two external fuel tanks (giving them longer range), a radiometer pod on the left wing and the two addition crew pallets in the cargo bay...

and of course, the instruments. Actually, the NOAA has replied to your question in their FAQs:

Why aren't NOAA's Hurricane Hunter planes torn apart in storm?
Planes are generally not destroyed by strong winds while in flight. Airliners routinely fly in jet streams with winds exceeding 150 mph over the U.S. during the winter. It's the shear, or sudden change in horizontal or vertical winds, that can destroy an aircraft, or cause its loss of control. That's why NOAA's Hurricane Hunter aircraft don't fly through tornadoes. In a like manner, NOAA pilots and crew routinely (but never casually) fly in the high-wind environment of the hurricane and don't fear it tearing the plane apart. However, they are always monitoring for &quot;hot spots&quot; of severe weather and shear that they can often identify on radar and avoid if it's too severe.