Connecting handheld to intercom in NORDO plane

Hey folks,

I'm buying an airplane with no electrical system. I want to outfit it
with portable coms. I have a Vertex VXA-300 handheld with headset
connector, a Lightspeed QFR Solo headset, and a David Clark DC10
headset. I haven't bought the intercom yet.

Is there an easy way to get all this to work together? Are there any
special features I need to look for in the intercom to make it work?
Will adding a PTT switch present a problem?

I'm getting some dual in a borrowed Luscombe with a portable intercom,
and for the life of me I can't figure out how to talk on the radio and
to my instructor at the same time. I suspect it's because the
intercom's not set up for connection to a handheld, but know little
about the subject.

Thanks,
Mike