Keith Ellis and the guys over at Ipswich Makerspace did brilliantly well with their robot, TractorBot, at last year Pi Wars. They won Best Robot under £75 and placed very respectably in most of the challenges. (You can see the full results here). They have decided to open-source their designs and code for TractorBot, which means that other competitors this year will have a head-start on many challenges if they take the time to go through the code. Design specs will also be released so if you want a good base to start from, you could do far worse than take a look! You can read the start of their open-sourcing project here.