If you want a .530 ball, order that size mold. The Shrinkage you worry about is taken into consideration by the mold maker. You have more to be concerned with relating to the purity of your lead, than you do with shrinkage from cooling castings.
Yes, get a scale. You don't have to spend big bucks on an electronic scale, although they are nice. You can buy a simple balance scale from RCBS, or Lyman, or several other sources. MidwayUSA seems to have a good price, and so does Midwest. Do some shopping, and you should be able to find a scale for $20-25. If you don't own a caliper, buy one. AGain, about $20.00 of you shop right. This you can use to measure the diameter of the cast balls for sorting by size. For plinking you can shoot just about anything, without sorting, as yardage is usually short. For target and hunting, you need to sort the balls by diameter and by weight, as both can change the POI.
I have both Lyman, and Lee molds. I have even used an all-brass mold once, although I no longer own it. The Lee double cavity mold is fine for your purposes.
You would have to really abuse a Lee Mold to ruin it. Its been done, of course. Even Lyman molds can be destroyed by the right idiot. Follow the instructions that come with the molds and maintain them between casts. The Lee is made from an aluminum alloy, not pure aluminum. Its is very tough stuff, but not as tough as the steel used in Lyman molds. I would not get rid of my lyman molds. I also won't be getting rid of my Lee molds either. I sold the brass mold because the handles were to short for my hands, and no matter what I used, I was getting burned. I got rid of the gun that fired the balls I made with the mold, and sold the mold at the same time. I would not hesitate to own a brass mold, again, but I would put handle extensions on it.
