I am in the middle of making a dio for a large Gundam -

he will be using a sniper rifle (yeah - I know -willing suspension of disbelief)

the problem is this - he's (it) is 1/100 and there is not much in 1/100 so I am wondering what would be the best scale for any buildings / structures in the dio. I could scratchbuild but as time is growing short on this campaign (Mecha Madness) I am trying to find shortcuts. HO (1/87) looks ok and N is just too damn small - works for 1/144 gundam but not the big boys. Any thoughts ?

HO train scale is 1/87. For what you are looking for, it would probably be close enough. There are almost an unlimited number of building kits, figures, and accessories in HO, check out the train section of your LHS or on-line retailers.