What should I upgrade first?
Switching from 3x8 to 1x11 would be an expensive upgrade, well beyond your budget unless you get lucky hunting for used equipment, and may require a cascade of upgrades you hadn't planned on, which increase the cost further.

You'd probably be better off saving up for a new bike--bike manufacturers are able to get parts at much lower cost, so when you buy a complete bike, you're getting the parts more cheaply than you could get them as an individual.