The insulated ones formerly used for frozen stuff get around some of the problems, but bring their own.
Why so oldyin,is it a contamination thing ?
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I’d imagine the sort of insulation already installed in a chilled container seriously reduces it’s internal volume to start with, and means you’ve got more material to cut through and dispose of should you want want to make more door or window openings, on the plus side it comes ready insulated and probably has some holes for pipework through which you could route the outlet from your wood burner.
I think on balance I’d prefer to build my own timber frame, insulate and kit it out as I see fit…
on a £/sq-ft basis I think a good timber shed is probably a bit cheaper, can be configured to suit the available space better, would be warmer (if insulated) and easily repaired/modified…
Edit:
http://www.shippingcontainersuk.com worth a look, the do used containers of all sorts and bespoke conversions…