Is 1990 ford E-350 A commercial vehicle?

If it's used for commercial purposes, it is. If not, it isn't. If you're asking for the sake of wanting to know whether or not it requires a CDL, the answer is no, unless it's either a bus designed to transport more than 15 persons (including the driver) or it will be carrying sufficient quantities of hazardous materials to require hazmat placards to be displayed. In either case, you would be required to obtain a Class C CDL with appropriate endorsements (P for a passenger bus, S if it's a school bus, H for hazmat).