Okay I worked it out myself. You
do need 1.7 times more experience to level up AND the stacks are 1.7 times bigger, but this actually makes sense as it means that there is still about enough enemies in the game to reach level 30.
I am in Greenwort still, level 2. My team consists of Archers, Priests, Horsemen (a whopping two of them), Peasants and Swordsmen. I don't think there are any other ranged units available this early?
Fireball is very useful for a Mage early on, but mana regeneration is SO SLOW. So I put a point into that when I could, as well as Scouting 1 and Distortion 1. Given that money is going to be very tight, I have decided that Trading 3 and Trophies 3 are going to be early priorities. I have always loved Trading and I refuse to sell anything until I have Trading 3, and I figured Trophies will help me out too. But I have to avoid getting anything killed. Maybe Stone Skin can help early on...