1) You usually would have milky traces on the oil dipstick. Sure the radiator was burped properly? Jack it up and let it run while continually topping up fluids.
2) Cracked head? Engine out, replace head. Headgasket? Engine out, replace all gaskets and clutch if its old (peace of mind)
3) Where are you located? Best bet is to go to a Subaru specialist and ask. You would be looking at a minimum of $1500 for a headgasket. Don't use a second hand headgasket.
4) As above, headgasket minimum of $1500 but you want to keep it longterm and the cambelt/water pump/clutch etc hasn't been done in a good while you'd be looking more towards 3k. Replacement engine would be easy but then no guarantees on it.