I'm in the process of building an HO scale sawmill also...The sawmill structure itself is a fairly small, backwoods operation...(approx. 50' x 25')
As far as the interior, I'm planning on making eevrything myself from odds & ends in my scrapbox...i. e. steam boiler, log carriage w/track, & circular saw blade(s)
I'd like to be able to use some really nice castings, but I can't bring myself to spend the money if I don't have to...I may re-think that though, if my homemade stuff turns out badly...wall1