If you replaced the hoses, compressor, drier, expansion valve and converted it to R134 it shouldn't be much over $1500. I would find a place that can identify the leaks and replace whats necessary till you need a compressor and then think about whether to change. Conversions work alright on that model but they don't do as good as an R12 system would.
I know many shops that only convert now. We did that for years but now only do it on certain systems we know work OK. Find a shop that will fix the leaks in R12 till the compressor is needed.
Bosch Master, ASE Master, L1
33 years MB technician