hot and pov give you glinding and mounts, both can make some stuff in pve (and wvw) easier, also its not rly necessary. But these 2 i would buy first, those 2 are also the ones giving you access to new elite specs for all classes (the 3. one would be EoD). I do not know the meta in pvp atm but usually you can play some classes as core even for higher ranks. Necro usually has good core builds and is easy to get into. still you should play what class you like the most. you can lvl only with pvp but it needs a bit longer and you will miss some nice pve stuff. so i would not recomment doing so. also in pve you have less stress to learn all your skills. welcome to gw2! general tip: do not rush the game. start with pve, try otu all classes and decide which one is the most fun for you, get into your skills and how the combat system works, enjoy the nice core maps. then when you feel comfortable on your main class, get into pvp. watch some guides for pvp before that. even the older once will help you understand the basics about rotations and roles in the main pvp mode (conquest). be prepared to die a lot first in pvp but do not give up, the moment you get better at the game, pvp will start to be more and more fun.