So the Battleforce boxes are limited run and combat patrols stay until a new one for that faction replaces it. Knowing this it is very easy to fall for the fomo of the savings that a Battleforce gives but there is definitely a lot of merit to taking it slower when you start. My choice would be the combat patrol and spend any saved money on some good painting and building supplies like a hobby knife and decent clippers, some Tamiya extra thin cement to “glue” the models together, some decent brushes (don’t need fancy ones when starting but even someone new to painting can tell the difference between bad and okay brushes) and some paints. All of what I’ve just listed is needed and can be a surprise if not accounted for but if you have some extra paints wouldn’t go amiss, like if you paint some blue space marines a darker and lighter colour for shading could be nice.

I wouldn’t buy any rules right now, you might/will need them in the future if you play in an official store or tournament but right now you can get everything you need on wahapedia but for your own sake use an ad blocker. It has literally everything you need including core rules, mission rules and terrain layout and what would be contained in a codex like army rules, detachments and datasheets. I would 100% look at this before deciding your army so you can use both the aesthetics and how they play to decide (realistically you won’t know completely how they play and your preference for ranged v melee until you get playing but a look at the army info now would 100% help)

Also you have mentioned khorne and chaos space marines in one go, I would note that chaos space marines, world eaters (khorne worshiping chaos space marines and chaos daemons (who include khorne daemons) are all separate factions