The shop I use (and trust) to work on my 55 charges $80/hour for labor...$3,500 is ~43 hours or one work-week. Given everything it takes to install all the pieces and parts and make them work, I think that's a reasonable estimate for the amount of time needed.
I did my own installation, and in terms of total hours it took me at least double that... but I had never done an AC install before. I bought a crimper so I could do my own hoses and fabricated a custom bracket to mount the condenser, which I painted the same color as my car. I also had to do substantial adjustment and fabrication of the compressor bracket in order to correct the angle and make the AC pulley line up properly with the other pulleys.
Along the way there are a ton of little things that pop up, so you need to be able to problem solve as you go. I bench-tested the fan unit before installation which made me realize it had a bent internal wall that had to be fixed (easily bent back into proper shape) before installation under the dash. I had an issue with the controller switch which turned out to be a bad crimp, so I fixed that by soldering the connections. Nothing crazy, just all things to figure out and deal with.
I did cut holes in my freshly painted dash because I wanted those beautiful chrome vents - which was a terrifying experience - that I pulled off by working carefully and using lots of tape...and "measure twice, cut once" methodology.
I mention all this to help you understand what you're getting into if you go the DIY route. If you are adept at working on cars and not afraid to tackle a good sized project, then you will be ok. Just go into it realizing that you'll learn along the way and will likely face some frustrations...but in the end you will prevail.
Finally, if you decide to DIY and have questions, obviously the people on this forum are amazing and helpful, lots of answers available here. You can also feel free to PM me, I'm happy to help if I can.
