Nice Italian frames from the big names are very expensive on ebay. A lot of times the condition is misrepresented. The smart thing to do would be to buy a complete bike then sell off the vintage parts to offset the cost of the frame. You can get a free frame this way if you shop well.

As for stiffness, if that's a priority you need to ride a few bikes to see what you think. High end vintage euro frames tend to feel alive under you, connected, smooth, fast, but not necessarily stiff as racers now are used to. Some people can't handle the flex, some don't mind it.