Are you sure your crock pot is 850-1000 watts? Most slow cookers are actually around 300 watts or so at most. If it's a dual-use sort of appliance it may have a much larger heating element and some sort of thermostatic control, in which case the higher wattage could well be correct, but very likely if it's a slow cooker with a crockery insert it is relatively low powered. Examine the label on the cooker to find out for sure.
If your crock pot does not have digital controls there is absolutely no reason to need a pure sine wave inverter for it.