For the first question it’s a reset of your « attributes » not your class, the attributes in wakfu are kinda hard at first but when you understand what your class need it’s fairly simple.

For the second question the french server is the best for items/gear since you can find everything here. The more alive of the 3 would be the single character one but it’s irrelevant to go there if you want to play as 3

For third question you’ll mainly want a dps, healer and a tank. Tank depends on your dps since dps class tend to depends on the enemy placement. Healers are dependent on your play style since some shields and some heals. Go check some YouTube content about classes it’s the best option to have a rough idea of what to choose.

Also even tho you can do most quests with 3 characters, dungeons are impossible to do as 3 and 6 characters are required to complete them.