I have done several 123's, and one 126 with the Sanden upgrade. The total cost for all the parts is less than $600. Yes you could get a new R4 from Compressorworks, but I doubt it will last more than a couple seasons. Before I started producing the Sanden retrofit bracket kits, I bought a new R4 from O'riley and installed it and had it charged. It failed moments after the charge was complete (done by a professional shop). I then got one from AutoZ..., and it was defective right out of the box. I decided I had had enough with the R4, and set out to make an adapter for a Sanden. The rest is history. I am biased here, but I will offer my opinion anyway
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The Sanden compressor is quieter, smoother, takes less horsepower, is more efficient, and costs less in the long run than an R4. My first Sanden upgrade was done on my former 84 SD using a $40 USED Sanden style compressor, and it is still working great today (my friend Tony loves this car!). As others have said (and I agree), keep the SD and fix the A/C. It will NOT cost anywhere near $3,000!.....Rich