Personally, as with what a few others have said here, I'd start with the featherweights and go to heavy events - I'm in no way an experienced roboteer (I have about 3 projects in the pipeline, one of which spontaneously combusted a few weeks ago, and no events under my belt yet) but featherweights are vastly more simple to build for newcomers to the sport I'd say - you can use drill motors for drive which come in at about £12 a piece for where I get them from (with some admittedly lacklustre batteries but they are usable) and again, you don't need to spend as much on armour because the thing's only smaller. Likewise with ESCs that don't need to be as beefy, so they can cost less (I think)

I've learned a lot from building my first featherweight, and I'm still learning as time goes on - my aim is also to build a heavyweight at some point, but the way the sport's evolved has meant that you need a fair bit of experience and money to get into the 'competitive' realm of heavyweights, not that that's to put you off, mind, just an observation of mine.

Still, it could be possible, you'll just need to design it well - for that, I'd say go to events or watch the videos on Youtube - there's loads of videos videos of various events, feathers and heavyweights alike. I'd suggest checking them all out and working it from there - counterdesigning against both flippers and spinners is something I've really had to focus on with my machine!

Good luck to you, whatever you decide to build!