Yeah that sounds kewl. Though if you get really old parts, it'll be like building a dinosaur in terms of oldness and new computers will look futuristic...
Especailly if someone gives you a old Pentium or Pentium 2, in which case, its a cartridge rather than a chip.
Still a good learning experience though. Good luck with it
heatsink* the fan on top of the cpu! and thermal paste btw for the cpu
leads? wtf r u talkin about? u mean LED's? pointless lights u dont need them
fans should come with ur case make sure u get a decent case if u have aim talk to me IM me at computergen7 and if msn then firstname.lastname@example.org
The only leads I can think you'll need are some IDE cables if your motherboard doesn't come with them, and possibly (though if its only a small based system not much use) some extra molex power connectors if the PSU doesn't come with enough.
Other than that, you should be set with leads.
If it doesn't come with enough fans on the case if there are any, you can easily buy some from computer shops for about 1-2 quid each, very cheap indeed! Usually 80mm ones on old cases, though 120mm ones are becoming the standard now which can spin slower and still push as much air, though I'd check on the case you want to get before buying