The SRX240 platform has been announced end of life so no new features will show up. Buying a SFP-module for the SRX240 will be very pricy as well so I would recommend looking at the SRX300 - and as a lab/test firewall it is good that the SRX300 is fanless :-)
Please note that you cannot buy support when buying second hand and that way get the latest software updates for the SRX300... and you need the latest version to get a stable platform as the initial releases was quite buggy :-)