I made a DIY version of the spring thing for my Kuwahara many years ago. That bike had loooong dropouts. The reason for the spring is so you can push the wheel forward to remove it. Not necessary on bikes like Susie with vertical dropouts.
The first thing to do is put the fender in a rough position to get a sense of how it's going to fit. You will probably need a spacer for the boss under the seat stay bridge, but maybe not. Make sure the stays are long enough for where you need to be. Your tire looks relatively svelte compared to what the frame can hold, but it does have light knobs, so I would try to mount the fender as close to the frame as you can.
The plastic SKS/Bluemels should be fine as long as they fit over the tire. One nice aspect of metal fenders is you can re-radius to get more coverage for really big tires. So if your Gus is 650B, you could buy a wide 700c and re-radius for the smaller wheel.
The main trick for fender installs is to avoid mounting them under too much pressure (though plastic are more forgiving in this regard.) Try to secure each contact point with the fender in its natural position, not pulled or pushed and kept in place by the hardware.
Final tip: If you haven't done so, chase out your fender bosses. They may be full of paint, and if you try to force a bolt, you might snap it off. Ask me how I know. Finding an M5 tapered tap locally might be difficult. I found a seller in Kentucky who shipped a pair out for $10 within a few days. Have fun!
Here is my latest fender install, last week. I was not really concerned with a great fender liine, as I consider these woodies to be mostly decorative, "chop" style, since they don't have the compound curve for full protection. Also, I will sometimes run this with bigger tires, and I want them to fit, but I will need to swap in v-brakes with longer arms (these are the Shimano DXR MX70s Riv sells and is out of stock on, I believe I measured around 110.), as this is the current sticking point for getting the fender line all nice and also as far from tire as possible.
