I understand that you have limited computer resources, but consider this. Two of the block sets of Dystopia at Daz are free,
http://www.daz3d.com/i/3d-models/-/dystopia-city-blocks?item=4706&_m=d
http://www.daz3d.com/i/3d-models/-/dystopia-city-blocks?item=4709&_m=d
You cannot get much cheaper than that. And seeing that you pay nothing for them, if they overpower your CPU, you have lost nothing for trying the products.

You may be able to use just the buildings, render a scene, and use the render as your background. Or some one with a more powerful computer could do this for you.

LMK

Probably edited for spelling, grammer, punctuation, or typos.