A couple of things on this subject, if I may...
1st - My humble opinion on the orange gunk is that it's sediment from mixing tap water with the coolant (which most everyone does). Why it turns orange? Dunno. Why it settles in your reservoir? Because that's where coolant sits and it has a chance to. It never circulates there. Nasty isn't it? As you may or may not be aware, coolant is sometimes pressured into the reservoir when the system is hot and then sometimes it's sucked out of the reservoir and back into the radiator when the system cools, to sort of "top things off." Your cleaning out your reservoir is an excellent idea if you don't want that crap getting sucked back into the radiator.
2nd - I have seen quite a few times where someone has not maintained their cooling system and, after 80,000 miles, they want a flush. Generally speaking, this is really risky. After this long, not having changed out the fluid, there are deposits in the cooling system that aid in sealing it. Flushing washes all of that out. Leaks will occur as the seals/gaskets have aged and worn. This also holds true for automatic transmissions and laquers that have built up (but that's another post). If coolant changes are performed routinely, then flushes aren't really necessary unless you just want to. But if someone is working on 70,000 miles and hasn't changed their coolant, then draining and refilling is ok, but flushing is not recommended.
Also, with coolant, don't buy into the "extended life" stuff thinking you won't have to change it out as often. Coolant has a shelf life of 18 month and will go a while longer. Why would it last longer under the duress of being in an engine? I'd recommend draining and refilling every two years or 24,000 miles, whichever comes first. 5 years or 150,000 miles is a bunch of hooey. The additives that make coolant what it is have done worn out.
Hope this helps.