Most if not all new mandolins come with coated strings like Elixers or EXP's.
Tone guards are great to have, but I've gotten tired of having a chunk of metal between me and my mandolin so mines in the closet...they do work and prevent your body from dampening the back of the mandolin. To see what I mean, hold your mandolin against your body and play it, then hold it away from your body and play it. If you like the latter, then get a tone guard.
As far as armrests go, Doug's McClung armrest are as valuable to me as my pick. I wouldn't play my mandolin w/o either, and Doug is a great guy to deal with.
And now for today's weather....sunny, with a chance of legs
"Give me six hours to chop down a tree and I will spend the first four sharpening the axe." - Abraham Lincoln