Peter,

First I am going to echo the comments made above, go for a featherweight to begin with. Featherweights and heavyweights are the two weight classes with the most events held. Antweights are popular as well.

In answer to your question, I they would probably be ok but I would use something like the bosch 400 or 750 or a similar motor in a middleweight. Technobots have a range of motors which may be useful.