I just started up a couple of weeks ago. Only really been playing a week as I took my time on the ISBoxer setup to make it expandable and adaptable for later levels.

I started 4 boxing but dropped back to three as I didn't like how much housekeeping there was with 4 chars.
I plan on picking up 3 mercs for the other spots.

My team of Bezerker (tank), Dirge (utility) and Beastlord (DPS) are only level 17 but it is fun so far.

I plan to pick up the following mercs:
Primary Healer - Inquisitor
Second healer - ? maybe templar
DPS - Ranger

I would just choose what you want for the first team, you can't go wrong with a Tank, Healer DPS though.

Warden is my favourite healer, with a Fury second. It really just depends on your healing style though -
Reactives (Templer, Inq)
Wards (Mystic, Defiler)
HoTs (Warden, Fury)

I am already planning my next team as this one doesn't have much "feeling" to it. I will be going:
Brawler - Tank
Fury - Healer
Swashbuckler - DPS
Pirate themed of course and wearing the pirate clothes from the Marketplace.

Have fun. This is a GIANT game with a lot of different systems. For instance I had forgotten that you need to level skills in this game like weapon skills - slashing, piercing and crushing. If you ignore one and get a great weapon of that type later on you need to do some work to get the skill up. I love it!