I do not mean to offend but to appraise an E23 car the appraiser needs to know how full is the gas tank.
I'm thinking those days are behind us. Truly nice 745s and some manual cars are bringing pretty strong money. As for 90% of E23s, I think the volume of fuel thing might still be true.
I've got some cars insured through American Collectors Insurance and haven't had to get an independent appraisal. They were a bit hesitant on my requested value on the B7 Turbo when I increased it before driving it to Monterey, but I provided some supporting documents of sales in Europe and they didn't have an issue.
I dealt with a third party appraiser on my E12 M535i after it was hit in my driveway. I thought I got a pretty good feel for the guy and finding out he owned a couple of '68 Glas GTs, I was thinking we were going to be in pretty good shape, but unfortunately, I think he was a shill for the insurance company and got everything we discussed mixed up. In the end, it did work out, but not thanks to him. That said, I'm sure it was a much different situation than dealing with someone that comes out and can put their hands on your car and can apply what he/she sees to a proper metric.