This idea you have can go one of 3 ways. Either you go all out, or you build a games store quality table, or you realize that you are in for more work than you can handle, so you quit.
For building an all out table (something to provoke envy from everyone), making most stuff from scratch is the best way to go. Styrofoam on plywood, cut, shaped and based to look like rubble, ruins, and craters. Then use plasticard and cities of death to build the city.
For a games store quality board, its a little easier. Get the 2x2 boards they offer in store, and go nuts. Its easy to find ready-made roads at hobby train stores, so all you have to do is cut off the roll, glue, and base around the edges. Right after christmas there are always little mats for sale that people use for their christmas villages. You still need the cities of death buildings, but they can be made on their own modular bases, so your city can change between each use.
Basically, are you going for this:
http://recon-by-fire.blogspot.ca/2010_04_01_archive.html
or this:
http://www.dakkadakka.com/gallery/515393-Table.html
or lastly, this?
http://www.dakkadakka.com/gallery/5252-Panics%20City%20of%20death%20with%20a%20nice%20patch%20of%20grass%20for%20a%20sandwich%20and%20a%20futuristic%20frappuccino%201.html