I say go for it.
I'd add a Nem before adding corals.... Nems will move around until they find a happy spot. If you already have a bunch of corals in the tank, the Nem can sting them in the process.
I've found BTA's to be extremely hardy/easy to care for. Far easier than many corals. My euphyllia seem to be much more finicky than my BTA. I added fish, then a BTA, then corals. Haven't lost a single thing yet.
Unless the Nem is very large, and/or the tank very small, a dead Nem will not nuke your tank. Also, its pretty easy to tell when a Nem is not doing well. You would have plenty of time to remove it if things go south. I would not let the fear of your tank being nuked stop you from gettting the Nem. The whole tank nuking thing I believe is greatly exaggerated.. Their 'stings' are pretty mild at best. Its not like they have a bunch of toxin in them.
Saying that, I wouldn't buy a dying/bleached Nem. Try to find a nice healthy one and you should be fine.