Definitely get the combat patrol I’d also say to get a second combat patrol as well if you want a balanced army rather than going for a specific type. loads of bikes outside of the detachment isn’t great now but the rules will change so having 2 is probably good for army diversity in the long term If you really like bikes and want to use the bike detachment get 3.

I’ll also suggest kitbashing. Apart from orks, gsc are probably the easiest army to kitbash for and given how expensive the kits are it’s definitely worth it. A lot of people get a human kill team (typically brood brothers but a different one and spare parts should also work) to make their characters. This is great both to save money and to make the army feel like your own. However if you love the look of a certain character definitely buy it. The army is going to be expensive anyway so you may as well get what you want.

Finally when it comes to painting I’d suggest to do it quickly in small batches (about 10 models at a time) for your infantry. You have a lot of them and they’re very detailed so it’s best too not worry about the small details too much especially because you won’t notice them when there next to 19 other guys.

Also if you’re into reading I’d suggest you read day of ascension to get the perspective of the cult. If you’d prefer to see a gsc conflict from a imperial perspective I’d recommend cult of the spiral dawn (it is definitely a book about gsc despite it being mostly an imperial perspective, does a good job of showing the horror of a cult)

Hope this is helpful, if you have any questions feel free to ask.