I’m not so sure I would agree that weight isn’t important (but then again I only way 165). A good Madison race is 10 miles of intervals – hard acceleration, relax, repeat. The bike has to accelerate or I will kill myself just getting up to speed (of course the handoff helps a lot too). Weight is important, but stiffness is even more important for this type of event. Flexy cranks and frame, and a heavy frame and wheels makes this type of event difficult (a missing & out is somewhat similar if less intense).
Frame geometry doesn’t seem to be the concern here, it is what type of frame you want. 58mm bottom bracket is fine. 68mm (road) bottom bracket drop is unusable. Most track bikes seem to be between 55 and 60mm drop.
That new langster is an interesting looking bike, but man does it feel heavy.
Realistically, for the first 2 years it doesn’t make too much difference what you ride. You need to learn the track, riding skills, and learn how to sprint well. After that point, the details of the bike start to make a difference. Its going to take that long to really know what you want/need in a track bike anyway.
Building your own bike is an expensive way to go compared to a bundled factory bike (although upgrading can be fun if you enjoy that).
Durability isn’t really an issue on a track bike. They are subject to no debris, no bumps, no water, and maybe just a few wrecks. Any new bike should last decades on the track and still look relatively new (crashes not withstanding).