Question:
Most of the meters are single-phase units, as shown. There are also a few three-phase meters installed ? generally only for the penthouse suites in each block. They are old meters ? not pulsed ? and would need to be replaced. I?m thing the A100C to replace the single-phase units, and the A1100 to replace the three-phase units.

Answer:
I would suggest the Elster A220 to replace the single phase, and Elster A1140 to replace the three phase, because we've done those in the past. If you want to give them Smart Metering - where they can switch off the supply? I would suggest you use the AS230 for single phase and AS1440 for 3 phase (up to 100A). With a KOCOS modem. (The A220 and AS1440 both talk RS485, so you'll need a converter for this). Direct multidrop is usually about 10-20 meters per modem.
20 in a panel? You can try 1 KoCoS modem per panel. We do sometimes have more, but then I'd suggest breaking it up with RS485 inbetween.