If you have dual zone climate control then you have 2 blend doors. Plus a re-circulation door. Good news is they probably don't need to be replaced -- odds are the sensor contact wiper inside the actuator needs to be cleaned. The actuator is greased at the factory and eventually some grease makes it's way onto the sensor. It tells the computer the position of the door and if the computer loses that signal, the door is locked in the current position.
The bad news is they are a b i t c h to access, at least on my 2004. Actually the driver's side one wasn't too bad, can be accessed from under the passenger side dash. But the passenger side one (and the recirc one) require complete disassembly of the dash to access them.
All that said, if your compressor clutch is not engaging even though the AC is turned on and the temperature is above freezing, then there's an issue in the cooling system. Intermittent operation like you describe may be as simple as excessive clearance in the compressor clutch. As they wear the gap increases. Eventually the gap is wider than the electromagnet can overcome. But a bump or facing uphill can give things just that extra little nudge so the clutch engages -- for a while. The gap can be adjusted, but requires a compressor clutch adjustment tool and may be too rusty to move.
As for AC pressure: AC pressure tells NOTHING about the state of charge. If there is ANY liquid in the system, even just a drop left, the pressure gauge will show the vapour pressure of the refrigerant at the ambient temperature and will be the same as if there was 20 ounces of liquid in there at that temperature. IE: pressure = temperature, not quantity. AC gauges should have temperature equivalents printed right on the face, or you can google a temperature/pressure conversion chart. The low pressure gauge should show the ambient air temperature when the system is off, and cycle down to no less than about 2?C (34?F) when the compressor is running. If it gets that low, then the compressor should kick off until the pressure rises to ~30?C. That is just about the ONLY thing a cheap low pressure gauge from a kit is good for.
Quantity can only be determined by weight. Evacuate the system and weigh how much is recovered, or fill with a known weight of refrigerant is the correct way to fill the system.
One can approximate the correct fill by comparing the temperature of the line going into the evaporator to the line coming out of the evaporator -- they should be equal when the compressor is running steady. If the line coming out is warmer then there is no residual liquid exiting the evaporator and the charge is probably too low. If the line and accumulator are frosted over, then it is probably overcharged and you run the risk of liquid getting to the compressor. Since liquid cannot be compressed, this qualifies as a Very Bad Thing.
Bear in mind that if you lost your refrigerant, there is a very good chance that you also lost your lubricating oil since it is carried around the system by the refrigerant. Charging the system indiscriminately could mean running the compressor dry and dry compressors don't last long. There are little testers that are pressed on the low pressure valve with the system running and can give an idea of the oil concentration in the refrigerant. In general, it is better to have too much lubricant than too little. Too much will reduce the cooling capacity, but at least the compressor will survive.
If the system pressure leaks down to 0psi, one must assume that moisture can enter and the system will require a long hard vacuum at least and if it sat at 0 for any length of time the accumulator (which contains the dessicant) should be replaced.
Finally, beware of kits marketed to do-it-yourselfers that contain sealants. They rarely work as advertised and some can ruin your system. The ones that claim to cure seal leaks aren't too bad, but any sealant that claims to fix holes in the metal parts generally leads to "Black Death" if it meets any moisture in the system and the entire system will have to be replaced.