There is nothing wrong with Dexcool as long as you maintain it properly. You can always flush it out after your 1 year warranty is over if you wish to. What I would be curious about is who is offering a 1yr warranty on that (junkyard?) motor... get that warranty in writing!!
The Dorman isn't perfect, but its better than the stock unit but not as good as a sleeved unit. I don't think a Dorman has failed yet. Properly Installed it should last many years. Remember that the stock units typically don't fail until they have upwards of 70k miles on them. Mine was at 140K miles when I replaced it for the very first time.
Lastly, if they haven't put the Dorman on yet, have them replace the lower intake manifold gaskets with the aluminum framed ones. They are a new design so you'll / they'll have to get them from the dealer. Well worth the extra $50 and half hour of labor.
98 SSEi 111,222 miles.
Intense PCM, LS6 Valve springs, PEMS, FWI. Hawk Street Pads.