How to disconnect air conditioner lines from outdoor unit

First off, I vote for a new unit if you can swing it, but $100 is a lot cheaper and you are not likely to offset the cost of a new unit with the savings from a newer, more efficient unit for many years.

The condenser fan motor isn't that hard to change, you just need basic hand tools and a gear puller. It would also be a good time to replace the fan start capacitor as well.

You are correct that the refrigerant is not a consumable. If a system is low on refrigerant, then it has a leak. The technician should have a "sniffer" tool that can detect refrigerant leaks. If they didn't use it last time, then they were irresponsible. Also, refrigerant is expensive so that attitude of "if it gets low again then we know you have a leak" is wasteful of your time and money.

If the compressor in the condenser (the outside unit) is running and the air is not cool, then you are probably low on refrigerant. Frost on the refrigerant is also a pretty clear sign of low refrigerant.

I am additionally concerned about the condensation on your duct work. Is the air handler inside the conditioned envelope of the house? If so, that could explain the condensation you are seeing now but would not be seeing normally.