If you make it a vector image then you can resize at will without things becoming nasty and pixelated. Should probably start with designing the roads first, then work buildings around the roads. Also, regardless of the file type, be sure you work at a high dpi and resolution. The filesize will be large, but it gives you a lot of room to fiddle with and makes fixing mistakes easier in the long run.

You definitely don't need to use Illustrator or even the pen tool, but you'll want to make sure to be careful with making layers and naming them. If everything's drawn on the same layer, you're kinda ****ed.