Also still playing, but I've never made it past 30 due to rampant altholicism.
GW2 classes generally aren't much like their GW1 equivalents, especially the mesmer. There are three mesmer playstyles I'm familiar with: 1. is to use your clones as a situational crowd control, 2. is to use confusion to make enemies slowly hurt themselves, and 3. is to use phantasms for relatively high DPS. Either way, you'll be squishy, but you'll have a lot of survivability against low damage mobs while downed thanks to your clones. You'll also get a teleport spell that groups enjoy. As much as I play the class, though, it can be frustrating when soloing as your satisfying skills take time to set up, and your clones are too squishy to give you the time you need, so combat devolves to running around in circles while trying to keep the mob off of you. Also, all your attacks are hot pink and sound annoying.
(Edit: I should add that the mesmer takes more technical ability and concentration to play well. It's not a class you should play if you're looking to zone out and button spam.)
I won't speak towards the necromancer as it's the one class I never figured out how to play effectively, but unlike GW1 you're very limited on how many minions you can have at a time (iirc 4 or 5 is the max).
As far as weapons go, each class has unique skills for the weapons they use, so the weapons you choose should be reflective of your playstyle. Almost all of the theorycrafting is in how you allot points to your skill tree.
If you're uncertain, go to the PvP lobby (Heart of the Mists). You'll be level 80 with max skill points and access to all weapons, and you can try out various playstyles versus the class bots in the north part of the region. This'll give you a chance to get a feel for each class and its weapons before you put time into building one in PvE.