Older weenie builds rock. For some reason, as soon as a bike gets to 20lb or below it turns into a flyer and its rider becomes a hero

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On a recent build I started with a 5lb frame and got to the magic 20lb mark, so would say a 531 frame would be dandy and let the components do the work. You can improve as you go, and as things turn up. Top areas for retro budget dieting are:
Rear gears: go for a screw on freewheel hub. You can still buy Maillard dural freewheels for under 30 clams and they weigh nowt saving loads of weight.
To go with your Maillard go for an old tubular wheelset: the fetch less money than clinchers especially if built round screw on freewheels.
Next: BB. Shell out on a ti bb and you'll win lots of grams; or plump for a hollow axle like the Axix Light from TA.
Bolts: Cheapo quill stem with an alloy wedge, ti long bolt and clamp bolt will shock you at how light it is. Also works on the seatpost clamp: cheapo Gipiemme & SR posts polish up a treat or go for Kalloy; add a Ti bolt.
Mechs: Suntour.
Chainsets seem to be much of a muchness at around 600g but drillium is supercool. Gipiemme are pretty light but have ISO tapers. How about a home made compact set up? A 110 BCD MTB set running only two rings. Old SR ones are 500g. (Watch your chainline though.)
Gear shifters: simple friction: 600ex, gipiemme etc
Head set: Stronglight: under a 100g and about £20. Utterly awesome build quality.
Brakes: suntour or 600.
Saddle: Tart up an old flight TI, or look out for a Rolls titanio that's badly listed (they have silver trim not gold).
Pedals: suntour superbe revived with new bearings, or cheapo new ones from VP or Wellgo are ~ 250g. Look Keo if you're new school.
Bars - 3ttt Super Competitzione can be had for under a tenner. Ambrosio too.