Now that I have a working rig (first rig), I'm interested in buying stages separately.
I bought a used Scubapro rig in decent shape and brought it to a recommended technician for service. I told him I was brand new and it was my first kit. I got it back with a leaking first stage (told it was OK, as long as air wasn't on too far in advance of dive), no second stage/octo parts replaced, and only a quick wiping down of the second stage internals.
I replaced that first stage with a pristine Scubapro MK10 that I bought from Couv. I breathed through it, checked the IP (rock solid), and have been diving with that reg ever since. But I wouldn't trust the 1st stage I got from a trained, certified, experienced, and recommended local tech. I'll likely buy more equipment from Couv in the not too distant future.