Tried to resist getting involved with this but it was far too tempting.

I would urge you to listen to the advice that everyone has given you- most of them have built their own robots and know just how difficult it is.

For a first robot go for a simple 4WD pusher using either drills or Gimson motors- it will work and is a great way to learn about building robots. I started (many years ago now) with Whirlpool which was exactly that- it was very reliable and ran in its various forms until 2010. Since then we have built 18 other feathers which have all fought and hopefully got a little better each time (with a couple of exceptions )

I will repeat what everyone else is saying so start simple and work your way up! You will learn loads more by doing it that way and will not waste loads of money on ideas that won't work.

So my advice is:
1. Keep the idea simple
2. Build the simple robot
3. Get to an event meet people and learn from the experience
4. Build on what you learn from fighting your first robot
5 Enjoy it!