The limit is commonly a flat plate above the burners and has 2 wires attached. It doesbt look like much because the switch is acually inside the furnace. Many times you clean and tap it and it will reset. If not it will need to be replaced.
The limit switch is a high temp switch in case of high temps in furnace from blocked filter, blocked register, bad blower. The pressure switch ( with the tubes) would not make contact if the outside sections of pipe were to get covered up.
Updraft means the furnace blows air straight up...like from a basement....down draft and horizontal are commonly found in attics
Photos will help greatly