I personally wear 5.11 pants almost on a daily basis, and I love them. One of the great things I have found with 5.11 is that as long as you buy the right size, they fit really well. I would be willing to bet that if you decided to go with 5.11, there would be very little alterations required. Only issue is they tend to have a lot of visible pockets on them.
If you go with 5.11, stick with their 2 or 4 way flex material pants, they tend to be a bit more fitted. I loved the Ridgeline pant they made, unfortunately I believe they discontinued those. I thought they would have been a great bottom for a 2 piece flight suit. You will want to stay away from their Ripstop BDU style pants. Just like traditional military BDU's, those are a little baggy.
Using 5.11 would be a rather expensive way to go, however.