The link will be helpful, but the only real thing for you will probably be the bottom bracket threading which is likely to be BC1.37x24 and the width which is probably 68mm or 70mm and the Shimano makes BBs that work with both widths for Shimano HollowTechII and older Shimano crank axles.

So as others suggested, pick your crankset first. If you stick with one manufacturer for everything you will probably have no issues. Your existing stuff is Shimano? Stay with Shimano and you'll have the fewest things to consider.

You can put an 11 speed on it and you will probably love it. I just put a full Shimano 105 5800 group on a 1991 Paramount and I am cursing myself for not doing it years ago. I've only been 7 speed with downtube shifters till now.

IMHO, ebay stuff is typically overpriced. You can find brand new for less from online retailers. Nashbar and Jenson are two of the four I frequently use. There are British sites that offer cheap prices too.