Hi P, Welcome to Permies!
The best advice I can give you is to acquire a copy of the
RMH Builders guide written by Ernie & Erica Wisner. This is the go to book for all new
rmh builders.
As Jason mentioned steel is not suitable for any long term rmh build. Metal can be and has been used, but in the long term it is not a good choice.
As you discovered yourself any build under 6" is not recommended for a first build.
It sounds like you have a small workshop with limited space ? Rather than a horizontal pipe system , consider building a brick bell instead. Much smaller foot print than a traditional mass .
Keep reading posts here at Permies and if you would like more technical info, then check out this site
http://donkey32.proboards.com/ they build some leading edge stoves over there.