Why would you have an estimate on that type of repair and not one for a metal repair ?
I think we have no real data on fiberglass repairs that were done in a similar fashion as most were probably not treated and sealed. The environment the car will be in is the biggest factor so any estimates on how long either will last..depends.
Aside from that - this might be the most practical way for this repair on this car. What would be an estimated cost of a metal repair on this be , assuming the person wouldn't do it themselves ?