Can't speak for heroics but I've started out with a druid tank, druid healer and 3 locks (talking about throwing yourself in at the deep end....) and have been taking down the TBC instances fine. Some harder than others of course - Mana Tombs for example has been slowly carving my soul into tiny pieces. It's all do-able if you work at it I guess.

With 3 x polymorph, nice AOE healing and a one button tank mash I think you'll be fine with heroics. Certainly would have been my choice in hindsight, put it that way....

Edit - I'll give you one tip, when you hit Outlands do lots and lots of Hellfire Citadel and Coilfang Reservoir instances. Do them til you can do your keybinds blindfolded - they're great learning instances. The reason I tell you this is that if your experience is anything like mine, Auchindon is going to hit you like a motherfucking sledgehammer.