How do I know what power inverter to buy?

Howdy,

I need to power my laptop (Dell Inspiron) and a printer (Konica Minolta MagicColor 5650) from a car freestyle. The advice I've been given is to use an inverter. The problem is at hand are loads of different inverters out there producing different voltage and power. How can i figure out which one will provide adequate (but not too much) power for my two tendency?

Thanks
Al