I didn't consider this as one of the options because in the new section will be a drive in door, 9' or 10' wide. Don't want to deal with what would be necessary to incorporate that into a below grade monolithic slab. Would at least have to pour a ramp down from the curbwall.
My flooding problems with Katrina occurred with equipment that was stored in Biloxi, MS. This shop is in Tupelo, MS, so I'm not really worried about the hurricane tidal impact, just normal rains, which can be heavy and flooding.
Thanks for the input, I think I'm going with the step up and potentially raising the existing floor if the step bothers me too much. This is also probably the least cost option.
Frank