From the looks of the wiring coming out of the PSU and their size, I would say you should assume a 350W at max. Does it even have a single PCIe power cable? Also, check the nameplate's +12V rating(s)
And yes, take the PC outside and blow the dirt out of it. Then, if the bottom has a vent, turn the PSU over so the fan is facing down. It won't fill up with dirt that way and will cool itself with fresh air rather than heated air from the PC.