Do any of those function between buildings? The distance is too far to realistically run a physical cable, and I haven't heard anything about HomePlug or MOCA being able to reliably work between two buildings.
HomePlug can work if all of the electrical is on the same circuit, so it all depends on how the wiring of these two buildings is set up, i.e., if there's one main panel and everything is essentially on a single meter and all tied to the same circuit, then maybe it would work. If they're on two different circuits, forget it.
And MOCA is a whole different concept, and require coax, so if there was a coax line out to the other building which you say is 60 feet away, yeah, you might be able to set that up.
Replacing the antennas on your router is probably not going to be a very tenable solution, and is likely to either decrease coverage in other areas, or if you're increasing gain significantly, is also going to increase noise and introduce signal distortion. And if the signal can't penetrate the metal building, you're likely not going to be able to get through unless you move your router/wireless AP significantly closer to the building anyway.
I'd suggest that you use another router closer to the building as a repeater, but this metal building construction tells me that probably would just be a waste of your money and time.
How does this workshop building get its power? If you're on cable, the only real solution may be to simply do another cable drop and pay for an additional cable modem to get coverage there. What about a 4G LTE/cellular wireless hotspot? Maybe that's the best solution. I mean, you're talking about a workshop, right? Should your dad be working and not playing on the internet (just kidding). But I think your solutions are going to be somewhat limited....
Now if you could run some CAT6 cable in a shielded underground run from your house to the workshop, that would be ideal. Then you could simply hook up a cheap old router and put it into AP mode and you'd have it made in the shade.....