Which modems can I use to connect to PRI and Enermax meters?

Of course, if you know what you're doing, you can use any modem, but for a lot of them you'll have to have an APN or do CSD call-in.
Of the APN-less GPRS modems, the more reliable modem for the Pri and Enermax would be the TC65. You CAN also use an Enfora for them - costs less but also less reliable - and you can PROBABLY also use a Maestro (it will be able to do Enermax, not 100% sure about the maestro) - the price for the maestro (WITH SMARTPACK) is somewhere in the middle.

We normally supply the TC65 and Enfora to people, but the maestro they usually buy direct from Trinity.

The reason why some people would get the Maestro is when you want to walk a road with the guys at Trinity Telecoms, which brings it into South Africa, and has good support and training.
The pitfall to watch out for is that the modem should PROBABLY be set for 2400 baud with the PRI, although there are installations out there set to 9600. Do you have the PRI meter programming software?
Using our software, you could set the RS232 ports on the modems for both kinds of meters to 8N1 (8 data bits, no parity, 1 stop bit); but I know some meter programming software needs the Enermax modem to be on 7E1 (7 data bits, even parity, 1 stop bit)

All the above are external modems with external power supplies.
You can also use the KOCOS Elster A1140 modem, but as an external modem (like with the kit you get when you buy it for the Elster A1700).
You can also use the SmarToo modem from TruTeq (set it up in the same way as a SmarTee); but it is probably the most expensive one.
If you have an APN-less GPRS modem not listed here, try to set it up as a Generic Active GPRS Modem: it could work.
One last note: with the old Enermax meters, I assumed you do have RS232 ports available on the meters. If you have Enermax meters without RS232 ports, you'll have to replace/refurbish them, which means you'll have to uninstall it and send it to the manufacturer to be upgraded. So you may as well replace them with a more modern & reliable meter. By this time, they might be out of calibration anyway.