Here's my list of sleeper parts - as defined by reliability, relative rarity, and obscurity resulting in a low price.

Lambert / Viscount cranks - just like TA or Stronglight cranks but with English threading and typically 1/3 of the price
Nervar cranks - same as the Lamberts
Simplex retro friction shifters, and the various Spidel and Mavic versions
SunTour V-GT Luxe rear derailleurs, Vx-GT derailleurs - best touring derailleurs ever
MAFAC cantilever brakes, MAFAC brake levers, MAFAC centerpulls - high resale value if they are complete and nicely restored, but easy to find
Shimano 500 centerpulls - surprisingly well made!
Shimano Light Action / Z series - an entry level group with a very nice, understated silver look and excellent mechanicals. Their light action ratchet shifters are second only to simplex retro frictions as the best friction shifters out there
Shimano oval chain - 1/8" chain originally made for cruiser bikes but looks awesome on a track bike, very sturdy and reliable

some modern bits that are great for restorations:

sunrace thumb shifters - a grant Peterson favorite, $10 a set and just as good as any vintage bits
Alex DA or Weinmann LP18 rims laced to Quando hubs - sub-$100 wheel set that looks right on a vintage build
Action Bicycles cork bar wrap - cheapest out there but look great. A little bit of lacquer over natural cork tape and you have the same look and feel as leather for sixth of the price

bonus - cant afford a Campagnolo Pista crankset? Get an old Strada set that's 144bcd and then grind off the inner chainring tabs. Once mounted on a bike, no one will know the difference.