Not familiar with fireplace inserts so I can't comment on that, but is it heavy? I just want to make sure you've considered whether the floor under your insert has the structural support to hold it (plus a stack of firebrick or whatever you decide to use) up without sagging or damage.
In our cabin in Alaska we always used old face-down metal highway signs on the floor under our
wood stoves, but the stoves all had cast iron feet so it was more a thermal-reflection thing to prevent thermal radiation from a cherry-red barrel stove from lighting the wooden floor on fire or -- more plausibly -- charring it.
As for your roof, the typical thing is to run the stovepipe through a manufactured collar unit that has flashing (to mesh with your metal roofing and keep
water out) and a couple of layers of metal with insulation between them to keep your stovepipe from lighting the roof on fire when it (your stovepipe) gets cherry red or melts during a chimney fire. I'm not sure how to improvise that if you don't want to use the manufactured unit. A lot of primitive cabins just run the stovepipe up through the hole and then chink the gap with stuffed fiberglass insulation -- but a lot of primitive cabins burn down the first time there's a chimney fire, too, so I don't recommend that.
