David,
I am not an expert but here is my 2 cents. Assuming that you are going to have the yard prep and spray the bottom, I would simply have them do the barrier coat in the process. If they are going to simply strip the old paint and you are not going to apply the bottom paint, I probably would not.
Your friend is right in a sense about the vinylester to an extent. I don’t believe the whole hull was laid up in vinylester but I think you have a 4 mm (.16 in) layer between the gelcoat and the hull layup which is there for the sole purpose for preventing blistering. Soda blasting should not hurt the gelcoat of the vinylester barrier layer. The boat is only 17 years old and has probably never had the bottom paint stripped. If it has, someone may have used a more aggressive approach and actually sanded through the gelcoat and vinylester at some point.
If the yard is willing, ask them to take the paint off and then let you and some of your friends talk a look at what is underneath. If it looks bad, go for the epoxy. If it looks OK then don’t.
The reason I would go for the full treatment if I were spending the money for a full bottom service is that I would not want to see a few blisters appear in a year or two and be kicking myself. On the other hand, I could really use a new GPS and perhaps the money would be best spent there.
Regards,
Karl