+1 on butted vs. plain gauge, especially for a heavy rider on anything but smooth pavement.
With small flange hubs, there's little functional difference 3x vs 4x. If the flanges are larger, the added spoke length can buy a bit of resiliency, which is why 4x/36h was popular BITD when folks were using LF hubs.
If the rider is especially strong, or there's lots of hills, 4x handles torque a bit better because it's a full tangent, but again the difference depends on flange size.
Too many rims to list, but look for a wider box section to handle wider tires better (if the frame allows) plus a bit of depth.
The rest is about the quality of the build. For a clyde, I'd up the rear wheel tension about 10% over what I usually build (if the rim can take it).
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