A well developed yearling will work fine. Buy from an honest outfit, ask about their breeding garantee, and a reference or two.
Check out the epds watching the bw epd, keep it under +3.5 for Angus if your heifers are 700lbs or better. If you want to keep daughters, get a milk epd in the +10 to +15 range. A Yearling value of about +60 or a little better should give you decent performance without affecting calving adversely.
As for the bull, check his feet, and legs to make sure he moves well. Look for a narrow head and smooth shoulders. Actual birth weight under 85 pounds to match the bw epd of 3.5 as mentioned above. Make sure he isn't overfat, or underfed. Ultrasound if it is available will tell you how fat the bull is, .25 inches is about right at 1300-1400 pounds.
Ask for a semen test, (or BSE)and expect to pay between $1800 and $2500 (U.S.) for a good quality bull.
Jason Trowbridge Southern Angus Farms Alberta Canada