as for buildings being flat Im hoping adding some more junk near them will help them look a little less flat.
Bad idea, IMHO. Junk can add detail but you have to use brush geometry and lighting (!) to make the architecture look any intersting. Try borders, non-rightangled walls, wall columns ect. Add different lighting with shades of yellow(warm) and blue(cold). Get photos of places that look cool and moody and copy bits of architecture.
What about the lights inside that one building? They look to tacky?
I could be mean now and say the inside looks like Wolfenstein 3D... The old Wolfenstein 3D.
sry, but the extreme (floor texture for the ceiling) texture choices make the "flatness" of the walls omnipresent. The layout is completely right-angeled too.
Updating all this to a decent looking Source engine map will be a lot of work and requires to completely overthink the architecture. I think it will be hard to just "add on" what's there.