There are many reasons a unit can freeze up. Insufficient airflow, is one cause. This can be caused by, as you guessed a blocked air filter, or an evaporator motor which is not running properly. A unit which is low on refrigerant can also cause the suction line piping to be covered in ice including the body of the compressor outside in the condensing unit. You said a hose had ice build up, I assume you meant one of the pipes? Sometimes, your expansion device, (whether it be a distributor/capillary tube style or thermostatic expansion device), can malfunction. If there is a blockage at the orifice of the distributor or the TX valve's power-head loses it's charge, then he suction line can frost/freeze at the air handler inside.

The suction line is the larger of the 2 pipes, the one covered in insulation. The smaller, exposed copper line is the liquid line. If that has frost build up on it let me know, and I will tell you why that one would have ice on it.

Where are you seeing the frost/ice on the pipe? It makes a world of difference on what is wrong.

you need freon approx 40 to 50 bucks a pound plus service call.