If you are going to buy them off the farm, then you first need to find a ranch you are comfortable dealing with, so you can make a long term relationship. You want to find an outfit who uses good doing genetics as feed it way expensive and you want them to be able to put the weight on as cheap as you can. HArd doing cattle eat and eat and don't grow.
Shrink can be a little tricky, some actually make the owner keep them off feed overnight, some way them off the truck and use that weight, and some use a certain % shrink from the original weight.
If it was me, I would want a sort, as when you put them in a pen, the different size cattle require different nutrients. Then when you go and sell them, hopefully they will be a little more even as well.