Looking for some advise/direction at getting into the world of fighting robotics.

Since first seeing robot wars as a kid I have wanted to get in to it all myself and now finally have the ability/time to take something on. I have a fairly decent theoretical understanding of how it all works from school/university but as i'm sure you'll agree theory and in practise and 2 completely different things.

Was wondering if you knew of/had any robots out there for sale that I could have a practise with and eventually begin to enter into some competitions. Nothing too expensive as I don't want to ruin anything or throw money away needlessly if I make a mess of it, but something that preferably works that I would be able to experiment on.

That may sound a bit vague, but hopefully you know what i'm on about.