Also don't plan for a level 31 build, plan for level 30, since you're most likely going to retire at 31.
Like others said, as an infantry player you're going to want to max out your athletics for as much agility as you have. And you're going to want more strength than agility (at least IMO). Having 15 agility is nice for infantry, 5 athletics and 6 is pretty close and not going to make or break your livelihood. And having that 1 more weapon master isn't going to be a huge difference either. However having 7 (or higher) power strike will certainly come in handy. Athletics is important, but iron flesh is a throw-away skill and would only put skills into it if you have leftovers.
For melee/infantrymen I'd say that strength is more important than agility (but wouldn't stack all in strength, you want AT LEAST 3 athletics/9 agility). I'd suggest at minimum go with 12 agility/3 athletics, and my preferred build would have 15 agility/5 athletics.