I'd think the biggest determining factor (at least for me it would be) would be how much of a down payment you can put on the land and how much the bank will finance.

"Here" the bank would require a significantly higher % down payment on the land than they would on a new cattle confinement with a good business plan.

While you say that the cattle barn wouldn't increase the head you'd feed it should either get you more yardage/head if you custom feed or have better growth/feed efficiency if you own the cattle yourself.

Another factor is how you buying 30 acres affects whether or not your dad and uncle will buy the farm.   If you buying 30 acres is the tipping point to get them to buy the rest and you'll get to farm theirs someday then that definitely has to affect your decision.

Good luck.