I'm looking at getting a half-way decent air blower. What are people's opinions on what sort of pressure and volume of air is needed? I have been able to find some references to pressures for coal forges but not much on charcoal, except that it needs 'less than coal'. How about coke?

I use an electric blower and just control the speed using a dimmer switch. Cheap and easy solution. There is as much blast as ever I shall want but I can turn it down to conserve fuel.

Same here. I used an electric blower from a washing machine until it fried on me after about two years. Since then I have been using a reclaimed air conditioning fan. I control the air with a blast gate, but the dimmer switch might be kinder to the motor.

Some squirrel cage fans require a certain amount of air moving across the fan motor in order to keep it cool. On these types of fans it is best to simply redirect the excess air somewhere else and let the blower run full. This can be accomplished by a T in the air line to the forge with a gate on the side that doesn't go into the fire. You can also redirect this exhaust air up your smoke stack to increase the draft.

Also keep in mind that "dimmer" switches used for lighting are not suitable, be sure to get a fan speed control "dimmer" typically used for ceiling fans and such.

Finally, you want to make sure the speed control switch used can handle the maximum amperage load of the motor.

A friend of mine uses a hairdryer with 2 speed settings for his home made coke-forge. He even uses it for small scale casting.
Aeneas