Thank you for your answer.
Establishing GSM/GPRS link between the PLCs is a good idea, quite low cost option and we have some experience on this type of solutions. I would still consider putting a VPN router between the "master" and Internet. This way I don't have to get a fixed IP for the PLC.
The SW Raven sounds interesting especially if the price can be halved. With the GSM networks we are getting outside of my "comfort zone" (if there is any). So could you explain a bit more about how to utilize the SW Raven in practice. I understand that in theory if we connect all the PLCs to a VLAN we can transfer data between them (of course the PC with a browser needs to be connected too). But for example, if I understood correctly all the PLCs are in this case hosting their own web page, how in practice can I access them with my web browser?