Right now I'm looking at this one. It's extremely cheap and looks like what I need.
It has low/high voltage protection 9.7/15.5 volts. If I recall correctly, someone said that when cranking the car... it drops to 8 volts. If I get a regulator, will that keep it running at ~12v the whole time? Everyone keeps saying "get a regulator!!!!" but I'm not sure why. Some people don't have them, and some people say their computers don't reboot when cranking either. And where would I get a regulator? I don't wanna build anything, I just need something to plug in and use.
it really depends on the behavior of the inverter at the various voltage ranges it will see. when your car is cranking the voltage can varry between just a couple volts and over 200 volts, depending on the car and conditions.
a regulator may be able to help, but i dont think you could get one to supply a loaded up 400W inverter on the cheap. im in the same situation you are and i decided to go with a second battery and an isolator. that should do the same job as a regulator, for the most part.