I’m going down this same rabbit hole as you are and recently upgraded to a 1x system with my 2016 10 speed full suspension bike. I was fortunate because I only had to buy a new chainring and could keep my rear cassette, shifter, and derailleur so it was a relatively cheap upgrade ($40). I now want to get new wheels, but will probably buy used to save some money. I say this because, given your situation and what you’re looking to buy, I’d suggest getting a new bike. If you’re replacing a 3×8, that means your hardtail is probably an older model. To go 1×11 with a good mix of Shimano SLX and XT you’re looking at $300-400 for parts, for a good new fork, that’s at least another $500+, and the same for wheels. You’ve then got nice new parts mixed in with an old hardtail frame. I think I would just take that ~ $1500 you’re going to eventually spend and save up for a new full suspension and keep the hardtail around as a backup (or buy a new hardtail if that’s what you really like). This scenario makes more sense if you’re scouring Craigslist or Ebay and buying used parts.
As for parts, I often buy from Amazon too as they get it to me quickly and have low prices. I always compare with some other go-to sites like Performance Bikes, Nashbar, SteepandCheap, Competitive Cyclist, Jensen, etc… Good luck