Gas and Oil Home Heating Furnaces - central air unit freezes up

I have a Comfortmaster central air/heat unit about 12 years
old. This is our second summer in the house. Last year the heating and cooling worked well. This cooling season I noticed some ice build at the inside unit on the smaller, uncovered of the two pipes leading from the unit outside heat pump. The programmable thermostat was set at 72F.... the unit ran constantly but gave off little cooling. On a cool day the unit was shut off and the area around the inside unit leaked water. I removed the cover and found ice had built up on the fins(an inch thick in some places) over the coils, The water on the floor was from the ice melting.
I cleaned and vacuumed the fins and tried again but after about 3 hours the water on the floor and the ice on the coils had returned. What do I do now?

clean and your coil is clean and you allowed at least an hour to defrost coil with the indoor fan running,Then you are low on refrigerant.PDF

The unit is about 13 years old. Had a tech come out who said that he saw rust on the tank and oil around it and said that it was probably leaking freon. He said if it worked ok last year but not this year then it was a big leak. He said to repair and fill the unit would be about $500. He then spent 45 mins estimating a new system at aroun $3000. He told me he would not just replace the outside unit but had to replace all the piping and inside unit also. Am I being "conned" or is this the only way to go?