I'm also gearing up for a 2018 NoBo AT through-hike. Shopping is an area in which I can offer informed advice. I'm not claiming to know the best possible prices on everything, but I can usually get close enough that it doesn't make much difference. Your big advantage is that you're starting early enough that you can wait until you find good sale prices.
Money doesn't have to be a problem on your "big 3" (pack, sleeping, shelter) because there is so much competition selling these items that you can find very good (though perhaps not excellent) gear that won't break the bank. I got my Granite Gear Crown 2 pack a couple months ago for $159.49 total cost (no tax, free delivery). It's 37 oz. including the removable lid, and I could pare it down to 34 oz. That sale's not going on now, but there ought to be plenty of opportunities to get a pack at a discount between now and your through-hike start. Your budget would be hurt by a Western Mountaineering sleeping bag, but the Klymit KSB 20° down bag is
regularly priced at $189.95, so a sale should get you an adequate bag to keep you warm on the AT. Personally I went for a synthetic bag because I'm tolerant of cold but dread the consequences of getting down wet on this abundantly rainy trail: Marmot Cloudbreak 30, for which I paid $126.71 total. It's a snug fit, but only 31 oz. And there's another sale on
now, so this bag is available again for the same price. I haven't got my tent yet, but I'm leaning toward a Naturehike Cloud UP 2, going for $104.50 on eBay. So my "big 3" total is under $400 and under 7 lbs. I could go with my existing blue CCF sleeping pad for no additional money, or splurge on a Klymit inflated pad for $39.99 (available
here).
Sales come and go with no particular rationale that I can discern. I'd expected outdoor gear on sale for July 4th, but nothing came of that. Yet I found my GSI Extreme Mess Kit this past week for $24.95 instead of the $36.95 list price, just by doing shopping searches every few days on Google, Amazon, and eBay. There
is one nearly-guaranteed discount time to shop: Black Friday (day after Thanksgiving). At that time, nearly everything is on sale
somewhere.
The biggest job for you is producing a list of items you might reasonably use on the trail. There is a vast selection, but mostly (after a lot of work) the list will break into items that are too heavy (most Kelty items, for example) or much too expensive (ZPacks stuff) for your budget. Of the remainder (decent weight, but still too dear) you keep checking the prices until a desirable item has a significant drop in price. Starting about half a year ago my pack list was the Gossamer Gear Mariposa, Granite Gear Crown V.C. 60, and ULA Circuit. I almost bought the Crown V.C. 60 when it went on sale for $149.95 (down from $199.95), but I found out the reason for that sale was they had introduced the new Crown 2 with pretty much all the Crown V.C. 60 issues addressed. So I revised my pack list with the Crown 2 at the top, and hit on a sale a couple months later.
Start building up your gear shopping list now, and check for sales: the InterNet is your friend!