I can't help you with a local sand blaster but as for sandblasting around the engine
and drive train, I'd say it will not be a perfect job around the engine and mounts. If they are using a normal size blaster, they will have problems getting the hose and gun around the mounts. Also the sand gets everywhere so the engine would have to be wrapped in plastic an cocooned very tightly. I would say pull the engine and drive if you want them to do a proper job.
As for how much damage is done to the bottom plates, I guess that depends on how solid the inside finishing was, and how long the water
sat there. I had an old HO-28 that has fresh water
standing for a year or so. It ate halfway thru the 1/8 bottom plates right at the joint between keel
and bottom plate. The water would evaporate down to a puddle then rain would fill it up again.