@ craig;

Brushless motors in large robots are still hit and miss at times... but when they do work its pretty obvious there far superiror to any brushed setup.... brushed still has the edge on reliability (at the moment... im working on it) lol...

Anyhow...

Brushless motors should be quite a good option for beetles, they have been proven to be reliable in this weight classs... but obviously to work out what motor you want, you need to know the following;

Weight of weapon
Gear ratio
Budget
Required performance.

If your on a budget and you disc/bar whatever isnt going to be to heavy... you could try a setup like this :
http://cgi.ebay.co.uk/2204-C2822-KV1200 ... 4aaba4fbd9

If you can spend a bit more, look at scorpion motors and castle creations esc's (two brands i rate highly. and use myself) Axi motors are also really good.

Remember... it wouldnt be a good idea to run your bar directly attached to the motor... you will want to belt drive it or something to take the shock away from the motor.