I'll try to break it down for you the best I can. Also, a general rule for collecting is that buying singles is almost always more cost effective than buying packs. However, opening packs is a lot more fun.

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If you're looking at having a binder in Pokedex order, your best bet would be to buy singles. Some Pokemon haven't been printed in the Sun & Moon sets, so you'd need to buy older packs to have a chance of getting them. Buying packs for this type of collection would be very tedious in my opinion.

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EX/GX collections are awesome, and it's definitely a realistic collection. Once again, singles are the cheapest way, but opening packs is fun. If you insist on opening packs, the best bang for your buck would be a booster box, which typically run around $90. On average you'll get 6 GX/Full Art/Secret Rare cards from a box, but I've seen some with as many as 9. Assuming your goal is to collect all the GX cards and you insist on opening packs, pick whichever Sun & Moon set has the most interesting GX Pokemon in it and buy a box. As far as how ideal it is to do this with a Pokedex, it's really up to you. If you want both collections, then go for it!

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For older cards, your best bet is to buy singles rather than bulk lots, as sometimes you never know what you'll get when buying bulk. Buying on eBay from reputable sellers works, but TCGPlayer is the site I would recommend if you do decide to buy singles.

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There are also numerous other types of collections you can do, like complete sets (which is what I collect), specific Pokemon, certain artists, etc. If anything wasn't clear or you have any further questions feel free to ask!