robot arm for 3d printing

Mmmm.... You're going to need a robot that has a decently strong programming language, and allows you to link the motion speed of the extruder nozzle in 3D (6D?) space to whatever signal controls the extrusion speed. My preference would be a KUKA or ABB.

Depending on just what kind of 3D printing you want to do, you might need to add external axes -- a long rail for the robot to ride on if you need to cover a long range, or possibly a rotating turntable to allow the robot to reach all sides of a narrow but tall object (like the columns in your linked video). Always better to actually add an appropriate external axis for the robot from the manufacturer, instead of some 3rd-party unit.

A robot in that payload range isn't going to be cheap, easily in the $75k range, and that's before you add the 3D printing hardware, and the Safety hardware that you absolutely should have.

Also, you're going to need a complete software toolchain to take a 3D model, slice it, then generate operational robot motion code. Unless you plan to have the robot act like a simple 3-axis 3D printer, you're going to need a fairly high-end robot simulation software package to make this work. Some of those packages, like ProcessSimulate, can run $20k/seat/year, and that's before you add brand-specific postprocessors. You might want to look into OctoPuz and RoboDK, or maybe Visual Components, but you'll need to make sure you price out the software with all the necessary add-ons.

A 3 axis gantry robot may be a better fit.

3000 mm height would be a challenge, given the vertical orientation of almost any extruder tool. Desiring a second hand robot suggests you possibly have a limited budget. Another challenge.

For really BIG workpieces, I concur with HawkME on the gantry suggestion.

I have seen Thermwood's Large Scale Additive Manufacturing machine up close. It is a large format gantry dispenser.

Thermwood LSAM - Large Scale Additive Manufacturing

It is used to dispense polymer structures and then machine the surfaces smooth. The machine uses Siemens PLC + CNC + servo drives. CNC path programs developed from CAD-CAM software (with additional customization). Servo drives for axis motion and large raw material melting extruder and smaller precision dispense extruder. Many man-years of engineering development work was done on this impressive beast. Very expensive. The last I heard (couple years ago), the US Government ordered so many LSAMs that Thermwood had a multi-year backlog.