Howdy! Getting started with 40k after a long time being in love with the lore.

Ive decided to go with Skitarii with the intention of eventually combining them with Blood Angels (it sounds like assault transports and vanguard go well together.)

So my plan at 500 points is currently just taking two squads of Rangers, one with arquebusses and one with pew pew lightning guns (arc rifles). For midrange/close I'll do a squad of vanguard with 1 (eventually 2) plasma caliper. Those troops will be supplemented by an onager with an icarus array because I like guns.

My question is how should I go about growing this force. I think I want to eventually have a three strong squad of icarus onager with cognis for iwnd.

Would it be better to do 2 squad of min Rangers with arquebusses and 1 squad max vanguard with callivers, or the reverse. Or just 1 and 1 and make room for some rust syalkers/infiltrators.

Is having the ability to drop pod and infiltrate more tactically valuable than just jamming more drop pods?

Are there any skitarii or blood angels formations I should keep in mind? Would any of the assassins be a particularly nice augmentation?

What would be a good starter force for Blood Angels to add in? It sounds like a librarian and some tactical marine in rhinos could be good.

Are pimped out onager worth it in 1500+ point games? I love the models and their sheer firepower, but I don't want to sink a ton of money into a unit that isn't going to go even on point value most times.

Sorry for the million questions, I'm just very excited and want to make sure I have a reasonable plan for spending on my first force.

Also: tips for painting skitarii? I plan to go with a ryza color scheme with bases with fine sand colored rusty to make rust dunes. Any suggestions for getting the darker rusted red for inside the coats? Any pieces I should paint first before assembly?