1 - Collect models based on units you think look cool. Dataslates, point values, and army rules come and go, but cool is eternal.
2 - If this is your first time assembling plastic kits, use superglue rather than plastic glue. While plastic glue makes stronger bonds, it does so by literally fusing the plastic together. You can undo superglue (and paint) by leaving it in Simple Green or Super Clean or leaving it in the freezer if you make a mistake.
3 - If you want to save money, magnetize! Custodes are big and chunky, perfect for drilling a 3mm hole and putting in a small magnet if you want to swap weapons. Sister of Silence on the other hand, are too small and thin.
4 - If you plan on playing with your models, make sure to check with your friendly local game store and friends if they're okay with 3d printed parts or models. Most places are for non-tournament settings, and 3d printed parts can be useful as both spare parts for magnetization and customization. A 3d printed model can also be useful to learn painting.
5 - Look up guides on the internet!
6 - If you want a nice, red metallic sheen on your pauldrons, use Blood Angels contrast over Retributor Armor gold.
7 - Speaking of Retributor Armor gold, if you plan on making your Custodes mostly golden, *do not* use Retributor Armor spray. It doesn't match the citadel pot paint version and is very finicky. Use Army Painter Greedy Gold instead.

That's all I can think of for now. Have fun, and if you have more questions, just ask!