Inductive loads like motors are typically not good things to attach to inverters; their initial current demand tends to be very high. You'll need a pretty big inverter, possibly as high as 1000W (unless it's one of those really little buffers). What's the amperage rating on the buffer's motor? How long do you plan to run it? Keep in mind that a car battery is designed to provide really high current for a very short period of time, e.g. for starting, before being charged again. If you deep-cycle a typical car battery, you can permanently damage or kill it in a relatively short period of time. Either get a deep-cycle AGM battery for your inverter, or plan to have the engine running so you're powering off the alternator (but wasting gas). Or, better, get (or rent) a generator.