W, to answer your question:

a. I had my entire system flushed out last year (including heater core) and refilled with the green antifreeze. I have since flushed it again this past November (I usually do it twice a year).
b. I used the Dorman UIM setup because, according to my mechanic, the aftermarket Dorman was just as durable as the Delco design, also it already came with all the necessary hardware, whereas with the Delco, I'd have to search for all the other materials (o-rings, gasket, etc).
c. I did not have the LIM gaskets changed as I was told the bottom half of the intake (including gaskets) was aluminum already, so there was no reason to change them.