I'm brand new to the game and wondering which of the 4 champions should I choose that will actually be worth having later into the game as well.

not sure if it's 2 late but, Kael in my opinion, best ALL round and good for poisen  for clanboss, athel being a close second though for later on use with giantslayer and weaken.

I think later game Athel is the most important starter for her abilities (weaken, 3 hit attack, fast cycle aoe), but she has terrible AI that drives me nuts.

Kael is very strong all around. 

I wouldn't look past these two. Elhain and Galek are strong in their own right (low level arena is so much Elhain), but they aren't as key to a strong party as Athel and Kael.

RuN!!