Building an effective rambot can be just as difficult and costly to engineer as any other robot.
You need decent batteries, ESC, drive train, weight distribution and traction.

For your first robot make whatever you're comfortable with and would find fun. Lots of people make lifters as their first I think because they are enough of a challenge to build and gives you an active weapon.

I'd just build something (within the rules), get it running, test it, fix faults and go to events. Learn, meet people, have fun and then think about the next one.