insulating is going to be a task .. for your side's and bottom I recomend RIDGID insulation(used under viynal siding) to help hold heat in, R- 19 is a heavy rating witch should do the trick .. but place plywood under ridgid to help absorb electric current witch could run through tank and zap fish and you even..
you may have to make some kind of wooden frame work to seal the glass to the side's . for as I have never seen or heard of anyone one sealing glass to metal , the expantion rate is so drastic I think trouble is on the rise...
incorporating your heater with your filter is cake provided you are useing a wet/dry sump style , get submersible's, two and place in your overflow chamber , doubleing up on the watt's mite be a good idea aswell
rock's as decor maybe out of the question , stainless still scratches !!! sand & driftwood may be your only option beside's bare...
and bro if you work in the steel industry some what you should be able to find a master weilder 25 +yr's experiance no problem , stainless is easyer then aluminium but can be phucked up as quickly too..
good luck
... post some progress pic's when able ....