All current Macs have more than enough power to drive a 3-5 year old 42" plasma.
The Plasma needs VGA or DVI input though. Without it, you are stranded.
iMac and all Macbooks can share the screens, yes.
Depending on the type of input on the plasma and the Mac you buy, a connection cable might be necessary, like "DVI-to-VGA adapter" or "Mini-DVI-to-DVI". Just check the inputs, then ask for a cable that supports either DVI or VGA for the Mac you choose in the store.