Yeah, like others have said, "time" and "money" are the things i need the most and have the least of, but in terms of more tangible components, I gotta say it's probably chains and spokes in the proper length.
To keep myself well-stocked in chains, I bought a huge run of 410-sized (1/2"x1/8") industrial chain, and it works/fits singlespeed bikes fine, but it's not as laterally stiff as the real-deal bike stuff. Despite normal chainline, I'm finding that the chain swings out and hits the tires whenever i go over substantial bumps. This is with adequate tension, a 1 piece crank, and a SS coaster hub.... yeah, the tire is a fairly fat 2.4" with aggressive knobs, but still.... I'll probably use the chain on bikes with skinnier rubber and/or bikes that spend most of their time on pavement, b/c the industrial chain ain't cuttin' it for klunk-duty....