For 1) That's going to be true of any resource that's used in spaceship construction. If you're building interstellar ships at such a scale that you might conceivably run out of stuff to build them out of, it's a fair assumption that you have the capability to go and mine the rest of the rocky planets in the solar system.
2) That 400 tons is not going to scale up with volume. Density of lead is around 650 lbs/ft3. Let's say you have a big ship, a mile in diameter. And let's be generous and put a 4 ft shell of lead around the whole thing. 4/3pi(5280^2-5286^2) * 650 * 1/2000 = 57000 tons of lead = 52000 metric tons. A drop in the bucket, tiny compared to the amount of steel and other material that would make up the rest of the structure. (Annual steel production is around 100 times more than of lead. We won't be running out of it either.)
The bottom line is that unless you're talking about transuranics or other extremely rare elements, or are building things at comically large scales (ie: thousands of ships) amount of construction material is not going to be a limiting factor in starship design.
2) That 400 tons is not going to scale up with volume. Density of lead is around 650 lbs/ft3. Let's say you have a big ship, a mile in diameter. And let's be generous and put a 4 ft shell of lead around the whole thing. 4/3pi(5280^2-5286^2) * 650 * 1/2000 = 57000 tons of lead = 52000 metric tons. A drop in the bucket, tiny compared to the amount of steel and other material that would make up the rest of the structure. (Annual steel production is around 100 times more than of lead. We won't be running out of it either.)
The bottom line is that unless you're talking about transuranics or other extremely rare elements, or are building things at comically large scales (ie: thousands of ships) amount of construction material is not going to be a limiting factor in starship design.