Hi Mike, welcome to the forum! I think Lucy has covered most of the important stuff here. I'd broadly agree with the comments on weight classes - I've spent ages trying to make an Antweight that I'm happy with, and I do find the fiddly teeny bits frustrating. This is speaking as someone whose other hobbies include painting Warhammer figures and playing the ukulele, so it's not just that I have giant clumsy hands. However if you want to just get something built and have access to a 3D printer there are loads of free STL files for Ants.

I haven't yet built a beetle but I get the impression that the "scene" is pretty competitive. That might make beetles less attractive to a complete beginner. That said, Robodojo do have a "Sportsman" beetle competition (i.e., no nasty spinners). There are also events like "Bodgebots" in Bristol. (I say "there are", what I mean is "there used to be and hopefully soon will be again!")

The first robot my friends and I built was a featherweight and I found that pretty good to start out with - there's lots of room for wiring and such, and the end result feels pretty satisfying. However, we had access to the Hackspace in Nottingham, which made the physical size of the robot less of an issue. If you are building at home and don't have a workshop, beetle is probably the way to go.

As far as building a chassis goes - I think most people tend to build a box, generally from HDPE sheet. You don't need to build a ladder frame or use spacers made from rods. Just cut out flat bits and fix together with barrel nuts like Lucy says, or take the gash solution that I used and just use loads of woodscrews.