We are very happy with the results in the end, and the designer is very skilled, got the project done quickly, was able correctly to make our changes, and had all the tools at his disposal to complete the job. There were a few bumps along the way. We gave the designer very specific instructions about what we needed at the beginning, including a CAD drawing of the room we wanted him to illustrate. It took a bit of back and forth before he actually included all the elements in the CAD drawing, and the things we had asked for in the narrative description. Some of the things he added on his own we had to ask him to remove. I think all this back and forth made him feel like he was doing extra work, but we felt like it was just getting us to the place we had described in the first place. Then we got fairly far along and realized the resolution was nowhere near high enough, so we had to pay more to basically have him start over. Hopefully we all learned a lesson from this - that it's very important at the very beginning to clarify what you will be using the illustrations for (in our case we wanted to be able to blow them up on large posters for presentations) as that determines the resolution, which, as we learned, needs to be set at the beginning of his process.
Again, in the end we are very happy with the results - he really captured what we were after. This was our first experience working this way, so we didn't really know what to expect. I would do it again applying the lessons learned.