1) don't use stop-leak
2) sounds like you have 2 problems, a leak on lower intake gasket, and another leak at heater core or heater core hoses......(just be sure you are not over pressurizing your cooling system due to bad head gasket!!!)
3) you don't have to do the head gaskets, but you will have to remove the valve covers to get the lower intake manifold off...

Buy a complete top end gasket kit. Replace all the rubber O rings(injectors, water tubes, etc..), save the head gaskets for another day....
If you do the head gaskets, you will need a few special sockets...like a short universal impact strength socket for rear head bolts, and an impact strength short flex bit for exhaust bolts... If your disco is rusty, plan on ALL of the exhaust bolts/nuts being tight, and possible breaking off a stud or two from the manifolds...