I would start at the fuel tank and check for cleanliness and flow. There should be a little fuel shut off in the base of the tank. There is also a filter in the shut off assembly. From there, there could be crud in the lines, dirty filters, worn injectors, etc. Those are easy (and free) things to check rather than spending money and throwing parts at an unknown problem.
Either way I'd change all fluids and filters regardless of what the seller said. That way you know what you have.