ok. So i'm just starting out.

Which class/classes are good for soloing?

Which class is the best to use to power level my other characters early on and how?

and anything else you feel is relevant

Most useful dps character is monk as he can provide the dps needed and his abilities buff and heal towers.

Most important builder character is a summoner. He provides the minions walls which are your tanks in this game.

If you have any specific questions feel free to send me a pm and i will answer as best i can.

Dungeon Defenders is an amazingly fun game with a great community. good place to start looking and learning is here.
http://forums.trendyent.com/forumdisplay.php?32-PC-Mac-(Ranked)-Trading-and-Grouping

From my limited understanding monks are better later on with good gear? Summoner I have no clue. I do plan on getting those classes later but how are they for power leveling early on? Or do those classes require good item and high level to be useful?

game really walks you through. as you progress you will find that eventually you'll need every char level and geared. Its tough to power level by yourself with any class besides dps.

Monks are pretty useful overall because of their tower boost ability. Later in game their dps will catch up to the heavier dps classes but i still find tower boost the most important aspect.

Monks are the most versatile fun class that i've stuck with the longest. And have many different ways to build and be fun.

However I think Squires are more appropriate first characters...tower squire to be exact.

They are usually good teammates and they can solo most maps by themselves better than most other classes dependent on just themselves.

I don't have a good one like I do my monk, but I can quite readily see why a lot of people love them, and they are very noob-friendly to start without becoming nearly useless toward the end like a lot of Apprentices and such.