Sketchup is a great piece of software for the price (free). If you haven't used 3d modeling programs before, it's easy to pick up and you won't reach its limitations for quite a while. There are plenty of add-ons and the program has a very nice 'warehouse' where you can essentially google search for models other people have made.
So say you want a 'car' in your model, type in the car you want, and out there in the interwebs is someone who spent 3 days making a detailed model of a model T Ford, and has shared it for free to input into your model.
We use this software sometimes at my office for quick and dirty visualization for clients, worth its weight in free gold.