well to be hoenst, it doesnt really matter, as long you enjoy playing your toons that is more important. you can e.g. play a healer live healer is always nice to have but if you hate that class there is no point in playing it.
as for PVP, as long your well super geared I think they all okay. you can be a dps if you dont have good gear and the opponent is a healer with nice gear you still get dropped.
I would advise you to get your own personal warcryer + BD if you go for a non nuker class. and a SE+EE +BD if going to be a nuker. SWS is a nice add on as well. if you dont want to box i think you should go for a summoner class (ES you can group with Nukers if needed, Warlock with melee) or you need to be loaded and use a lot of melody scrolls.
as for partys dont except to able to find random parties unless you in a clan, expect to be grouping with your own toons alot. so it is best to make your own party and join in other people parties when you up to level with your clan mates or other way around. if you make your own group. you be more flexible as well if you want to group and you able to swap in and out your own toons that are needed. with you clanmates. and run multiple parties.