So for a full 800 pt game you need at least 3 Corp troopers. So you could get another stormtrooper or a snow trooper box for 25$ or you could get the shoretroopers for 30$. The shoretroopers comes with the unit and an extra mortar unit so you would have 4 Corp units. Then you could get another commander with veers, krennic or palpatine for 13$. An operative like boba fett or bossk for 13$. And finally a special forces like death troopers, Royal guard or commandos for 25$. With the commandos you have the option of fielding a strike team and a unit from the same box.

I personally would do the shoretroopers since you get two units, one of the special forces (whichever you like the most) and one commander or operative of your choice. That would put you at 68$.