Hi! Welcome on board to the world of indie :)

Shiro is a great start! I've never used up a sample bag from them, because I either decide I need the colour and buy it, or don't and forget about it, but I'd say you could easily get 5-7 applications, probably more, closer to 10 from a sample.

Yes, you will need brushes, but they don't need to break the bank! I use these and I really like them, plus you can probably find them at most stores near you instead of Amazon. I use the smallest one for darker shadow as liner nearly every day, in fact, I'm wearing it now! I'd go for them over the Shany ones, there's less but they're all you need and a lot of us here can vouch for quality, whereas I've heard mixed things on Shany.

Primer is a necessity for loose shadows, and a glitter base is recommended. The ELF one you've linked is more of a glitter base, I've heard good things about it from people here but I can't vouch for it myself. A glitter base will make your shimmery or duochrome shadows adhere nicely, and look more shiny and vibrant, a primer will intensify your shadows as well, but it won't make things look quite as dramatic, however it will help them apply more evenly, fade evenly and last hours longer. For primer I've heard good things about Nyx High Definition primer. If you were going to have one or the other, I'd say a primer is more important than a glitter adherent, but both are good if you can swing it.

I'm still investigating the world of finishing powder, but a quick search of this sub for 'shiro powder' seems to contain the consensus that a lot of us here really like it for oil control. Once again, I can't vouch for it myself though.

Tutorials are abound around this part of the internet! Remember that to follow a tutorial you don't need the exactly product they're using for eyeshadow, just a similar colour and finish. That's how I get around tutorials, as indie makeup gets less coverage online. I'm on mobile, but I'll link some for you later! :)

Hope this helps, feel free to PM me any time if you run into more questions further down the track, or post again because we're always happy to help.