Sacrificial Squad are the four robots I was planning on building for my brothers event. There are designed around having lots of chewy MDF parts that can be easily replaced. These are the 4 robots:

Robot 1 : Vertical circular saw
Robot 2 : Twin hole saws
Robot 3 : Crusher
Robot 4 : Pneumatic Flipper

I am going for four completely different looking robots. Robot 2 was going to be an axe (Avenger) but it is probably not a good idea for several reasons.

The first thing I need is the drive train. They will need to be slow as I am expecting the people who would drive them to hopeless. So what I was thinking was 2 of these
http://secure.oatleyelectronics.com//product_info.php?products_id=578&osCsid=531459312e7ef01cba5ec731e6ce4074
with 150mm wheels. The 12V and low current draw makes it look reliable and able to run all day on one battery. I just need to buy a pair and find a good way of getting a wheel to stick.

coolies. we have those ones a plenty at work. if you get the power window type instead of those youll find that they have a large diameter outlet socket like the ride on kiddie car gearboxes that would probably be easier to attach stuff too.

go the wood crusher. they are cool
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www.demon50s.com - Minimoto parts
http://www.youtube.com/user/HyzerGlen - Videoooozzz

Those items are basically wiper motors. They would probably be a good choice for the task although you don't want them to be so slow that they get boring. Perhaps you can get wiper motors even cheaper from a local wrecker's yard?
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Australian 2015 Featherweight champion
UK 2016 Gladiator champion

My local wreakers are pretty good in the rip off area.
They seem to be the same as the setup in Bender, which seems perfect what I want to do.

The crusher would be the hardest to make but I do have all the parts. I just need to decide on the style of crusher. Since Hellcat is going to be used the other 3 robots need to be invertible. I did this scribble a few months ago.