if you just want to market your business on this forum then buying a Cooper membership imo is enough.
This is not enough. A brand new account really do not get much attention for anything they want to do in the forum. The best practice will be to hire a reputed campaign manager and launch a signature campaign. They can also buy advertising space. I know few projects who successfully used the traffic they got from the advertising buying on this forum.
I agree with you mate that buying only a copper membership is not enough cause someone want to launch a signature campaign they need to hire a reputable/ trusted member here in forum. Because if they are launching alone for sure some user will be ignore that signature campaign but if they will hire a trusted and reputable manager for sure many user will apply and want to promote their project.