Now, you find your major leak or wait til its in your budget.
I'd find out if it works by adding some refrigerant. If it leaks so fast you can't fill it, then its time for that decision. If the system works with some refrigerant I would suggest finding a shop that uses florescent dye testing (almost all A/C independent specialists use it) to add the refrigerant and dye. You stand to loose the price of the refrigerant if its a big leak but the florescent indication will give proof verification of all leak diagnostics, especially evaporators. If there is a real leak from the evap the dye trail will cover the automatic tranny where the evap water drains. If its a small leak, it will be verified on disassembly, not a good prospect once you are in there but better to know than not.