What scale capacity do I need for AC and heat-pump service?

For most work a 70–100 kg scale is enough. A disposable or refillable cylinder holding 10–12 kg of refrigerant plus the cylinder tare is typically 25–40 kg, so 100 kg leaves headroom for larger cylinders and recovery. For large commercial systems or charging from 50–60 kg cylinders choose 150 kg models.

What's the difference between a programmable (dosing) scale and a plain read-out scale?

A plain scale only displays mass — you meter the charge manually while watching the readout. A programmable scale has a solenoid valve: you set the target refrigerant mass and the unit shuts off flow once it's reached. This prevents overcharge and is convenient for precise charges on small inverter splits.

Is the scale suitable for flammable A2L and A3 refrigerants (R32, R290)?

The weighing platform itself usually is, but for dosing models with a solenoid valve check the datasheet for flammable-refrigerant approval — the valve and electronics must be rated for R32/R290. When working with A2L/A3 keep ventilation, avoid ignition sources and use accessories that meet the manufacturer's requirements.