Build a featherweight. Easy enough to build two and compete in a competition. You'll learn everything you need to know to think about tackling something bigger.

The biggest issue with heavyweights is that the mistakes are around 8 times the cost of featherweights (the costs tend to scale same as the weight). Mistakes will happen. Making these in the smaller weight classes will be easier. It also gets you familiar with the flow of an event, what happens at each stage and get you talking to roboteers in person.