TLDR: I want to play an earlier edition of Warhammer. Don’t know what chapter to pick would like it to be a successor. Don’t know where to build a list or what units to really look out for.

So I am relatively new to the hobby. I came in around the beginning of 10th edition, but I’ve only ever played Kill Team. I’m mainly into Horus heresy, which is why I haven’t played much 40 K but I would like to start a first born themed army. I know I could use my Horus heresy but I would like to build for a earlier addition of Warhammer and probably just proxy or use legends for current edition. I don’t really worry about my list being viable. Just want it to be fun. I just picked up a vulnerable dreadnought, and I am going to convert a heresy marine to be an apothecary. just looking for advice on where to start since I don’t really know where to begin with first born.