hi and welcome to the forum rob
pics wont be allowed for 7 days annoying but does stop spammers which i think is a good idea
if you know your model of wood fire we can get a pic and post it up till then your more than welcome to post in another part of the forum and discuss there your intentions and upgrades to your system
you can use mild steel but ss used in the right places makes a system much hotter where needed and is less weight than mild steel
there has already been some conversions via the standared wood burner they are on here using the secondary burn idea