Peter, which city, exactly? Modeling a specific city is challenging. The first thing is to decide what elements define the city to eyes of a viewer. If you're doing Salt Lake, for example, you have to model Temple Square for anyone to believe it's Salt Lake. Most cities in Utah have exceptionally wide downtown streets as well so you have to plan for those. Are there any other distinctive buildings? If so, go through the Walthers catalog and see if any are close. Kibri, Faller, and Heljan make non-descript glass and steel skyscrapers that will fit with most cities. For older structures, City Classics makes a nice assortment of four and five story buildings that will go with just about any downtown. Summit is now making modern backdrop kits like strip malls and national chains to bring things up to today's era. Even for a city you live in, the most important thing is to either take as many pictures as possible or use what's on the web. You'll soon see that the city has a certain look, as does the horizon, or backdrop, for our purposes. Pick out the buildings that define the city and the backdrop that will look like your standing in downtown looking out at the horizon. With some selective kitbashing and maybe a few scratchbuilt structures to duplicate centerpiece buildings, you can compress a pretty good size downtown in just a few HO scale blocks, which don't have to be more than three feet long.
