They're pewter, and just a soft alloy of some lead-free metals.
A dremel is a no-go, its just too damn fast to clean off mold lines. Trust me on this, one slipup with that tool, and you will gouge the model past the point of recovery.
What you want to have is a hobby knife, a set of files or sanding sponges, and lots of patience.
The mold lines are removed by keeping the knife perpendicular to the mold line and scraping away from you, down the model. The blade will scrape off the high point of the mold line and make it smooth. For really bad mold lines (some of my Iron Wind Battletech models, even the Museum Scale had some wicked mold slippage), sanding sticks or sanding sponges, or hobby files make quick work of the lines in hard to reach areas.
The only other thing about metal minis is that the larger, multi-piece minis might require "pinning" to reinforce the joint when glue is used.
I find a loupe like this very useful. https://www.amazon.com/gp/product/B005FZ1CDE
Also something to hold your work - https://www.amazon.com/Citadel-Outils-Poignee-peintre-mk2/dp/B08MC88KWZ
sanding sticks - https://www.amazon.com/Keystone-Cushioned-Abrasive-Sanding-Woodworking/dp/B00C5RFIXK
And some super glue and debonder
And don't forget to go easy on yourself and have a good time!
https://www.amazon.com/gp/aw/d/B00C5RFIXK?psc=1&ref=ppx_pop_mob_b_asin_image Comes with many different grits so you can make it real smooth.