Best advice I can give you is to wait for a final determination until you do a compression test. Have new spark plugs ready (great time to swap them in, given you are doing all the labor anyway, and with the age and mileage, a good idea).
I would hazard a guess that you might just have contaminated coolant and no large head gasket problem per se. There are other additives people put in the coolant inadvertently. Unless you have done all your own oil changes, its possible that a tech spilled something in the coolant (even a few drops of tranny fluid, oil or brake fluid will cause the irridescent coating on the surface of the water). Some coolant additives break down over time and will show up like this as well - something that the prior owner might have done rather than you.
I would also check out the intake manifold, which particularly if you have one of the all-nylon versions is prone to problems. The original owner MIGHT have had this upgrade/repair done as well, and some oil could well have migrated to the coolant at that time.
It would be unusual (though certainly not impossible) for you to have a tiny oil leak that was one way, ie, that never allowed coolant to mix with the oil as well as the reverse. If the car runs well and monitoring the oil shows no contamination, this is a clue.
Has the oil in the coolant re-appeared after flushing out the cooling system and topping up with all new coolant and water?
Finally, if you do elect to do the gasket upgrade, it IS a good bit of work, particularly with the engine in the car. Access is tight up against the firewall, and the exhaust will probably resist you at every turn.
Buy a good manual (Ford has the best, but the Chilton or Haynes are better than nothing) and read through the chapters involved.
I would grade it a 8 on the pita meter, maybe an 8.2.
"You can never be free until you let yourself go."