I am also looking forward having an ETV 300 one day. The problem is, if you have only the ETV 300 box with/out monitor and not the related typewirter (typically ET 121, 201, 221, 225, but also ET 111, 112, 115, 116, but mostly ET 121 as this was the cheapest combination). This is because usually the ET series typewriters do not have the optional serial interface (LCU, line communication unit) for ETV series. This is really difficult, because these LCUs can have different firmwares, for ETV communication, for telecomunication (Teletex), printing, external "DU 116" floppy drive communication, etc.). So even if you have an LCU, it might have the wrong firmware. I have an ET 116 with "LCU 116" (I got that LCU separated from another auction, in fact that was the first LCU which I have seen ever in eBay), this is 2x serial interface for printing purpose, but not for ETV communication. The serial interface hardware is the same, but protocol, escape sequences etc to communicate with the ETV are not public documentated, and I have my doubts if Ivrea would hand out it. The LCU itself fort all ET xxx series are little computers having their own Z80, Z80SIO/PIO or NEC 780x CPU+peripherial chips and they have their own firmware, depending on the purpose of the LCU model number.
So if you want a fully working ETV 300, 350, 500, you have allmost only chance to get it working if you get WITH the typewriter which was originally sold and configured with the ETV. If you buy a separated ET with serial interface with the correct ETV LCU firmware, next thing you should do is playing Lotto as you are a lucky guy.
So those peoples which kept the ETV 300/350/500 for some purpose and threw away the related typewriters are just stupid. Without the typewriter these ETVs unfortunatelly have no usage as the typewriter is the keyboard (and printer) for them.
So currently, if you like to use an ETV as a fully working CP/M system, better get the ETV 250, there, typewriter and computer is in one chassis, on one mainboard, but both parts of the system still communicate with an onboard interface to each other. I have one and playing the zork adventures onto it makes fun. Other software is difficult as the ETV uses an unknown terminal emulation, I haven't found any software yet which is compatible with more than the ASCII terminal settings. See my galery for the ETV 250
http://www.vcfed.org/forum/album.php?u=33579
Anyhow, be happy that you have a working ETV 300, they are quite rare today.