I don't know what the parts prices are like in the States, but in the UK, the brake discs are less than Ã‚Â£15 each, so it isn't really worth the cost or effort skimming rusty ones like those in the ebay auction.
Dan, what's wrong with another parts car? Lots and lots of goodies for minimal cost! (The parts car for my Volvo Amazon daily driver cost only Ã‚Â£70, which was Ã‚Â£10 less than a replacement windscreen, and yes, the screen on it was in perfect condition
It should supply all the bit you'll need for the brake conversion (or serviceable exchange units), and if you get a 1275, you'll also get the later separate master cylinders (I dislike the paired clutch / brake master cylinder, purely for engineering reasons) and of course, a 1275 engine & 'box