What is the best mod combination for old ben

At first, skip leia. Second, I think a great deal of potency and some speed is nice for old-Ben. After that I would say protection.

Why anyone would just dole out advice to not level x level y when there's no viable information concerning the OP's roster is beyond me.

Increasing Leia's critical chance and critical damage is useful. You wanna maximize those triple taps.

Ben
Lando
Han ST
Ani
Emp Palp

Thats a solid team

Speed and potency. I recommend you team him with STH, Rey and/or Yoda. Foresight gives TM from Ben's lead. Pre-palp/Lando, I would run OB (L), QGJ, Rey, STH, Yoda. Almost ever single fight Han would taunt and Yoda would give the entire team Foresight and Tenacity up. Also, Yoda gets TM from his basic as well. But then again, some fights I would have to give team BM from Yoda 1st if facing Phasma or if the enemy team took Rey's foresight, so as to give Rey that positive buff she needs to go ham.

imo Leia needs Admiral Snackbar to really shine.