A 12,000 BTU unit will transfer 12000 BTUs per hour if it is properly matched to an evaporator that was designed for it. Also, 12,000 BTU's is what is defined as one ton of air conditioning. In terms of cooling a CPU this is analogous to using a Howitzer to kill a mosquito.
One watt is equivalent to 3.14 BTU/HR. The maximum you would expect to see from the latest severly overclocked CPUs is probably no more than 75 watts (236 BTU/Hr). Since I work for an Electric Utility I certainly encourage you to proceed with this project (it will help my job security).
Also if you intend to use this unit you will need freon, a vacuum pump, soldering gear, a guage set and control circuitry. The evaporator will need an expansion valve or capillary tube. Other things like an accumulator/dryer and service valves will also be needed. How do you intend to build an evaporator that is matched to the unit and actually cool your CPU with it? In other words, the $120.00 might sound very seductive for that much energy transfer capibility but the practicality of actually utilizing it is zilch. The cost of constructing and operating it will many times pay for a dual processor machine with the fastest CPUs now available. Oh yeah, you will have to mount the condensing unit outside and run tubing to the condensing unit. It also probably needs a 220 volt 15 amp supply.
If these are used units and and have not been sealed, it is probable that internally they are in less than desireable condition. My suggestion is to stay away from it.
