Worrying !!! It looks like hit and miss whether I can get a custom built machine.
While no direct help, I would suggest going to the PrPro>Hardware Forum, and looking for Harm Millaards articles on building an NLE. He has several, focused on different aspects of the machine. I'd make a list of his suggestions, and then stop by your DAW shop with the list, and ask them to look over the specs. and report back to you, with their comfort level doing such a machine with those specs.
I had similar some years back as my guru had gotten out of the business and moved several states away. I could do it, but would need to spend a lot of time bringing myself up to speed, and wanted to pay an expert to do the work instead. I interviewed five well-recommended shops that were near by. Each had major issues with the specs. of my workstation. It was easy to tell that they were out of their comfort range and knew nothing about a graphics workstation for PS, or PrPro. Their suggestions were all wrong. My wife's hospital was doing some heavy-duty 3D neuro imaging, so she had the lead Dr. call. I told him what I wanted, and he gave me the name of his builder. In five minutes on the phone, I knew that he knew what I needed, understood imaging, video and light 3D workstations. Over the next month, we worked out the details, and he made me a very good proposal.
Took delivery and have had to make one tweak, and have then added some stuff over time. That workstation has been a stout performer from delivery, and is still going strong.
I did not have Harm's articles back then. Otherwise, I would have just handed off my distillation of those, and asked for maybe an update here, or there, as hardware does change.
If your go-to guy knows his way around a high-end system, and is willing to work with Harm's spec.s and recs., he might be your best source.