Sliders, track ends or EBB?
Builder choice
Geometry + fit
Tubes – material and dimensions
fittings
If it was my bike to be ordered tomorrow, for what little use that may be –
Bushnell EBB, or paragon / black cat pivot dropouts.
Builder, not sure.. Curtis, Shand, Mather? Someone with good MTB and/or tandem experience if it was an EBB frame. May depend on how much geometry experience / help you need and what their build style is.
Angles, verging on slack but not lazy, low BB.
853 main tubes, but 4130 cr-mo would be fine too. For me, something a bit stiffer than a stock 31.8mm top tube if the forks are 120mm or more.
Bottle bosses – 2 on down tube, 1 over + 1 under, none on seat tube. Rack eyelets on the dropouts just in case, but no seatstay bosses.
Colour – easy. Gloss black every time.
And for the kind of riding ss/alfine works well for (imo), I’d go for 100mm forks and 29″ wheels these days. That’s assuming this is to be a long-distance bike. But be careful if having a custom as your first 29er )