What are you planning to build?
if we don't know what you are planning to do with them, we can only say that both are good for you to buy.
Any motor will work with Arduino. The motorshield is dependent on the motor you want to control and not with the arduino. The arduino can even control an industrial motor, if you connect both to an AC drive. Again, if we knew what you are planning to do, we could say a bit more about it.
If you don't know voltages... I'd learn a bit more about them and then start up whatever you're planning to do.
Examples for Duemilanove work in Uno and vice-versa. Some examples need additional shields to work. Some shields can be connected to the Arduino Mega... most don't. So the Uno may be a better choice.
If you want to add sensors, depends on the type of sensors... it is always possible to add more. Are you ok, adding more chips to your setuo? Or do you want the least amount of hardware possible? What type of output do the sensors have?
These questions are not stupid... but, they show how much you don't know about what you want to do. And because of that, it is also very difficult to give some valuable advice.