The US Mortar platoon is a great choice to take with you American Rifle or Armored Rifle company. It will provide your GIs with instant support with both HE and smoke bombardments. 

US Mortars are cheap! You get 6 tubes and all the smoke and high explosive you can throw. You don’t need any specialist observers because you have the Under Command special rule. You can range in your mortars in with any command team in your force!