Drilling is the norm for saltwater tanks to accommodate the plumbing. If the bottom of the tank has been drilled, I would suspect it was drilled at the factory and at one time had a weir. The bottom is usually tempered and once tempered, it cannot be drilled, so the factory drills it first, them tempers it, then builds the tank. Sidewalls are usually not tempered so the owner is able to drill them. This is what is known as "reef ready". These tanks are plugged all the time since there are better ways to do this. It does not compromise the tank integrity at all. However, I would remove the plugs and silicone a piece of glass over the holes, then the conversion is permanent. Have him fill it and have it ready with water when you arrive so you can assess for leaks. Also if it is bottom drilled and plugged, it will not fit on a stand unless the stand has cutouts/space for the plugs.
Current Tank: 125g reef ready with 40 gallon sump. ATI 4x80 watt T5 with 2x Super Flora T5 Giesemann, 1x ATI Purple Plus, 1x ATI AquaBlue Special. GLA CO2 system, Flourite substrate, Digital Aquatics Controller.