The Nexus is a superheavy Combat Ship. It has a large amount of cargo space, a high un-Afterburnt top speed, and lots of armor. Hardpoints for 3 front guns, 2 launchers and 3 turrets make sure anyone attacking you is in for a bad day. A Nexus is, however, slow to accelerate (using the afterburner helps) and hard to maneuvre.
The Nexus has 6 gun mounts, more than any other user ownable ship in the game. It also has thicker shields (up to level 6) and bigger engines (level 10) than any other. Consequently, fully equipping a Nexus is more expensive than all other ships. After buying it, it costs another ≈3M to max it out. Once you do, however, you can mostly ignore hostile Talons, your turrets and shields will shrug them off without you even bothering to change course.
How to Counter this Ship
You can fight a hostile Nexus in a Mercenary mission; try the Flyboy Maneuver if your ship reaches at least 700 KPS. Boarding it confronts you with 4 crew members. To also take all of its cargo afterwards (if it's full) you either need to be flying a Galaxy or another Nexus. (Both are tricky, but possible: in a Galaxy you must lower your shields to power a fat rear gun; in a Nexus you must not ever let the opponent come too close.)
Where to Buy This Ship
For 1.8M available at these shipyards:
A mod exists that changes the interior design of the ship a bit to make it look nicer.