you have to give up your minimum requirements.
a charger have a maximum power output and current output. but when the power limit is hit first you will never get the maximum power.
it means the combination of cells and capacity will give you the information you need.

6 cells ok.
always charging minimum at 1C.
for example: 5000 mah -> 5A
required power: P= U*I*CELLS -> 4.2V*5A*6cells = 126Watt when you charge at 1C

if you want to charge at 2C, nothing wrong with that, you will need 250Watt and 10A charger.
you can calculate it with your own situation.

in my opinion, never buy a charger with built in 220->12V converter, the charge capacity is very limited.
especially when you go with more than 2 cells, you need really a powerful station.