What does 650 oz-in holding torque mean in practice?

It is the torque the motor resists while stopped with both phases energized. It is not the running torque, which falls as step speed increases, so size it from the torque-speed curve for your load.

What driver does this motor need?

It is a bipolar two-phase motor, so it needs a bipolar chopper driver rated above its phase current. The OMHT series datasheet lists the rated current and wiring; match the driver to those figures.

Is it a drop-in replacement for a NEMA 23 motor?

No. NEMA 34 is a larger frame with different mounting holes and a bigger shaft, so brackets and couplings must change. It offers much more torque than a NEMA 23 in exchange for the size.

Does it come with a cable, driver or encoder?

It is sold as the motor alone. Leads, driver, encoder and mounting hardware are separate, so plan for those in your build.

Single shaft or double shaft?

This version has a single shaft. That suits direct coupling to a screw or pulley; a rear shaft is only needed if you plan to add a handwheel or encoder.