To answer your questions:
1) I would go for a Xanura CTX35 or the newer PMIX35. The difference between these devices is that the CTX35 is an RS232 device whereas the PMIX35 is a USB device. Furthermore, the PMIX35 supports an extended protocol that has features for powerline diagnostics (voltage level, noise level, etc.)
2) Yes, the Rfxcom LAN interface + serial module can be used as a virtual com port over IP. The CTX35 works well in this setup: several members of this forum are using it in that way and report success. See also http://www.domoticaforum.eu/topic.asp?TOPIC_ID=487
. Note that there may be cheaper alternatives if all you need is a virtual com port over IP. See alo http://www.domoticaforum.eu/topic.asp?TOPIC_ID=893
. The additional benefit of using the CTX35 in this way, is that you may virtualize your whole home automation setup. Note that you cannot connect the PMIX35 to the Rfxcom LAN interface, as it lacks an RS232 serial port.
3) See 1).