New to Arduino -- Which board for a tour guide robot

I am new to arduino as well as this forum but I think this will be the best place to start.

I am about to start building a robot that you can tell where to go on my campus and it finds the building. This project is in the very early stages, found idea last week but I am considering using arduino per suggestion by a friend and I just am not sure what the right choice is. the plan I am currently considering is using map-building and localization based off of rfid signals.

Is there a particular board for robotics?
will I need a wifi shield? speech module?
Is there a size/weight limitation to what can be controlled with arduino boards?
how powerful of a motor can a board control because if this is an outdoor use project it will need to be a little sturdier build?
what motors are compatible?

I realize these are general questions and very basic I am just not sure where to begin. If anyone has parts suggestions, or even links to articles or research i should be reading it would be much appreciated. Posts will become more specific as I get started with the build. I am fine on the programming side it is the hardware that is new territory to me. Thank you in advance for any feedback.

Perhaps you can do the map building on a PC or an embedded PC and use the Arduino for sensors and motor control with a motor driver.

For anything at big, I would go with a Mega. Might I suggest using a GPS shield to get to the buildings. If you can get you hands on some heavy duty motors and a driver, such as say, some wheel chair motors, you will be good. Maybe even a robotic Segway, that would be awesome.

Thank you for your reply. I was wondering What would be the benefits I would gain from using the mega? As this is for a college project cost is a pretty significant concern so would using RFID instead of GPS be an appropriate trade off especially since using GPS would limit my usable area to outdoor only correct?

What would be the benefits I would gain from using the mega?

A lot more pins to work with! More pins = more parts to add-on and expand.]\

would using RFID instead of GPS be an appropriate trade off

Why not include both? Erdin looks as though he has a good suggestion.

Erdin:
Perhaps you can do the map building on a PC or an embedded PC and use the Arduino for sensors and motor control with a motor driver.

After further research I agree and intend to use the mega. Thank you HazardsMind for the suggestion. Also Erdin I apologize for the late response however I looked into more map building info and you are correct. I will be using the computer to handle that process. Now I need to go figure out what components I need and what shields will be appropriate with the mega. I also need to plan out a motor control setup. If anyone has more suggestions or ideas feel free to post them!