I purchased a 2002 Ford Taurus, little knowing that it had a head gasket leak. After it was driven for a week or so it started overheating. I am assuming that the person that I bought it from put something in it to clog it up before the sale as it was running fine. Then my dad put something in the cooling system to try to clog it, which didn't really work, and finally I had a mechanic try to professionally seal it with no luck. It ended up being diagnosed with a cracked rear head from all the overheating. I am going to replace the head but I don't want the cooling system to get clogged up when it's exposed to air? I am wondering what I should use to flush out all these sealers, if it'll hurt my engine doing so (cracked head), and if this product would work "Irontite ThoroFlush"? Any advice??
Your cooling system isn't going to get clogged up because of exposure to air, and you really don't need to do anything before the repair. Because you have a problem with the seal between your oil and cooling systems anything you add could transfer from one to the other, and you don't want chemical flushes getting in your oil. Adding the flush now could remove sealing compounds and make the problem much worse, not good if you plan to drive the car before you get it repaired. Flushes are also not good on the seals you want to preserver, unless you're going to get both gaskets replaced it's very likely to do more harm than good.
During the head replacement the mechanic is going to drain the cooling system, and then after tearing down the engine will get rid of all traces of old sealers and gasket material in order to make a good seal with the new gasket. Once the engine is back together they will refill the cooling system, if you have concerns that there's gunk in there then ask for a flush to be done then.