I suggest to go online, download the installation manual for the furnace or air handler, and determine the drain configuration.

Look at the specs for the pump you intend to buy; should have ample 'lift' (greater than 9').

You could size it by eye, as they are designed to empty when full. Take careful note of the wiring of the old unit, as some pumps have extra wires (beside power), wired to the furnace, to shut it off if the pump will not empty (safety switch). If yours is so equipped, good idea to replace with same type.

If you have a high efficiency furnace (90+%), you want an acid resistant pump.

V

Well, upon further experimentation it appears that I don't need a new pump. When I fill it with water, it does work.

The question now is whether the pvc pipe that drained into it was blocked (doesn't appear to have been) and whether replacing it will solve the problem.

It broke when I was trying to get the pump out of the tight space it was in, so it must be replaced and I can't identify any other leaks, but....