On dxing.com, the differences between the Hammarlund HQ-180 and -180A are
listed as "solid state power supply, 230 VAC, 3 position BFO, accessory
socket, less drift and 500 ohm line output".

How many of these are of any particular importance?

For example, what does the "3 position BFO" do for me that a plain (I
assume "single position") BFO would not do?

What is the accessory socket for?

Is "less drift" a dramatic difference, i.e. does the vanilla 180 suffer
from stability problems that the -180A fixed?

How much extra would you pay for a -180A over a -180 if both were in
excellent condition?