It was slightly warm yesterday, so I thought I'd turn on my air conditioning for the first time this year. I soon noticed the air through the vents was the same temperature as the outside air.

I popped my hood and noticed the outside of the compressor wasn't spinning. When I got home, I put one of those free-with-the-can gauges on the A/C low side nozzle, and it barely registered a zero. As soon I pressed the button, the pressure shot up to about 50 psi (the "stop filling" zone) then fell back down.

I remember having great air conditioning last summer, once I got the blower motor wiring fixed. Assuming this gauge isn't junk (which it might be), where do I go from here?

put some more refrigerant in it with the car off and in a quiet area and look and listen for leaks, sometimes a bottle with a mist pattern with water and dishsoap helps, spray around unions and welded joints thats the most common areas for leaks.

I bought a quality gauge: zero psi. I started filling the system, and as soon as it hit 35 psi, the compressor clutch kicked in. I charged it up to 50 psi (compressor off, 25 psi on) and the system was blowing ambient-temperature air.

I turned the engine off and heard a hissing with a strange smell from (I think) the front center of the car. A few hours later, the compressor wouldn't engage.

Unsurprisingly, it looks like a leak, maybe the condenser. How simple would it be to replace the condenser (assuming it'* bad)?

not bad to do most of the time you tilt the rad and there is enough room to get the core out without draining the coolant, never done a bonnie though so not 100%. but you need a vacuum to evacuate the air a few times before recharging. i usually purge it with nitrogen and suck it down a couple times then if it holds vacuum for a while charge it back up.