For the server:
src/net/common/servergame.cpp
src/net/common/servergamestate.cpp
I would start here as the servergame itself is managed here.
src/net/common/serverlobbythread.cpp
Maybe worth a look too.
For the client:
I never dived into the client/GUI so far ... but:
src/gui/
src/engine/
Here the client/GUI actions take place.
You have to figure out how to get a player back into the game - the server keeps the players until end of game - so that should be possible. I'd start with serverside-only coding, bringing a player automatically back when he's on 0 maybe. That should be easy to test/simulate.
For the rebuy-action the network protocol hast to be extended by extra "commands". These have to be implemented both on server and client side.
I hope that helps.