A lot of 28mm stuff (historical, not over-the-top fantasy) works just fine with 1/72. I think I'm right in saying that wargames buildings tend to be a bit undersized as it is, which means that 28mm buildings can look about right with 25mm-tall figures (i.e.. 1/72).
We played a 1/72 game the other day using a few
Dave Graffam card buildings that we often use for 28mm skirmish games. They looked just fine. You can also print the files out at different sizes.
I don't know if you're already familiar with Dave Graffam's stuff, but it's regularly on sale at DriveThruRPG/Wargames Vault for $0.99 a file. Each set tends to contain options and different 'skins' for each building, so you can get a lot of variety. There's a free building or two on the site, so you could print one out and assemble it and see how it looks.
For ruins, though, I'd tend to go with making them yourself. Cardboard, a hot-glue gun and some mod podge, and you're good to go. It really doesn't take long.