Thinking about options I guess I'd suggest some middle ground, which has also been suggested in another post. Here's my exact process, with the only mistake coming toward the end when I was fitting everything and becoming increasingly frustrated, and also lacking in the tools, knowledge and skills to find the leak.
I found a Chinese knockoff Sanden compressor on eBay for about $150 shipped (a place in Dallas, I think). Also got a generic dryer from eBay for $25 as well as a parallel flow condenser for $100 or so. I doubt the quality is top notch, as I had to get my local AC shop to do some work on the condenser flanges, but they all tested OK. Then I got new barrier hoses, fittings & o-rings from the same shop, which were dang expensive but necessary for this retromod. I also flushed the evap core with a bottle I rented from Auto Zone. That took hours before the flush came out clean. Even then I went for another half dozen or so passes just to make sure. Finally, I installed everything myself and even tried to charge it using my neighbor's manifold rig, but had a leak that I couldn't chase down and threw in the towel after a couple months' of weekends in frustration because I couldn't drive in this glorious early fall weather.
I did NOT replace the expansion valve. And now I have to, because I broke the probe lead with all of the on and off and on and off fittings trying to find the leak. I also managed to cross thread a fitting on the condenser and the compressor with all the messing around. The shop had to repair both.
My plan now is to wait until spring then see if the shop can't find the leak and charge it up for me. Maybe flush the entire system again with a pro rig while we're at it. I've already ruined the drier since I hooked it up with a leak in the system, so why not. Once you crack the seal you have about 15 minutes before you need to get it under vacuum. Otherwise it's toast.
Anyway, I still say have a pro shop do the final steps, if nothing else. You can save a bundle sourcing the parts yourself, especially if you don't need concours approved components. Since my car is just a toy, I don't care if it's A++. I just want it to work decently so that I can drive in some degree of comfort during the summer when it's 90+ degrees.
as good as a car can be... briefly.