Yeah... AC systems are one of those things where "the right tools for the job" cost a whole lot of money. :/ Leaking coils are not easy to deal with on your own, and to properly purge a refrigerant system once air has leaked in, you need a pretty stout vacuum pump. Air in the lines significantly reduces efficiency, and running low on refrigerant long enough can fry a compressor, as they're lubricated by the refrigerant (which air won't do).
You might want to look into your local power company incentives with regards to AC units or heat pumps. In a lot of areas, the power companies will pay you a chunk of the cost if you install a new, more efficient unit. If this unit is a decade old, it might qualify.
Of course, if your climate is somewhat mild, you might be able to replace the unit with a heat pump and save a good bit on power for heat going forward. That's not quite as cheap, but the newer heat pumps are pretty well awesome.