I am working on my first bike project (a fixed gear) and am trying to figure out what size bottom bracket I need. I know it is 68mm in width, but I don't know how long the spindle needs to be.
Do I buy wheels first and then a bottom bracket/crank to match that or vice versa? I obviously need to make sure my chainline is straight.

The frame I purchased is an old Paris Sport, and the front/rear dropout widths are 100/120 mm respectively.

Thanks!