End to End ? its designed for horses so its really not that fun to ride in one go because the gradients and surfaces are not ideal for riding quickly. Id ride the peaks 200 instead for a similar challenge with better trails .
Have you looked at Selle Anatomica ? I got a newfangled plastic one a while back and I think its very comfortable (Much better than B17s I've ridden in the past) also I think it would suit the aesthetic of this build.
50 mm knobblies rubbed the paint off the chainstays on my space horse. 50mm slicks might just be ok , I would take an educated guess that max clearance is something like 48-49mm
Re GPS , I would say that A wahoo element Bolt would be a good starting point , simple and intuitive to use , I have used various computers over the years and the Etrex's mentioned before are great and robust but a little annoying to use where as the Wahoo offerings are very straight forward.
End to End ? its designed for horses so its really not that fun to ride in one go because the gradients and surfaces are not ideal for riding quickly. Id ride the peaks 200 instead for a similar challenge with better trails .