First off don't listen to people that tell you what to play. Unfourtnatly all WoW classes are completly viable,some are easier then others, but they all work fine.
its more of what you like.
And yea, the problem is is that i well... havn't really played any of the classes(cept teh mage) past level 20.
to answer your questions, yea spellcasters can still cast spells no matter how close your enemy is. They only have a maximum range.
Hunters have a min range of 5 yards.
basically hunters are the least squishy, they wear mail armor. So they are a bit better at taking hits but do physical damage as a downside. The hunter also has a pet, which can be great for damage output(depending on spec)
Warlocks and Mages are extreemly squishy with cloth armor and low health pools but their spells(like all magic damage) ignore armor reduction damage.
Mages are well.. mages, they invoke the energy of fire spells frost spells and arcane spells. Their big thing is Strong AoE and Strong Single target. Their weakness is their squishyness.
Warlocks use magic of shadow and fire. They have pets which are all allright, none are preticularly good except the felhunter and felgaurd. Warlocks huge specialty other then mage spell casting and summoning deamons are "DoTs"
so they do a crowd control then put a bunch of DoTs on the target and hope the target dies.
So yea.. they are all kinda squishy, and good at kiting.
If you have any more questions( as its hard to answer this way) add me on an IM i have EVERYTHING.
AIM-Friagawolf
MSN-Blazingwolf2@hotmail.com
Yahoo-Cinderwolf732@yahoo.com
gmail-Blizzerawolf@gmail.com
So yea i can help you that way then.