Opinion needed about 2005 v6 suspension

You should be fine using the Mustang with the stock suspension, there are equally bad roads in the US that Ford had to consider. The V6 convertible is not known for its handling, but an aftermarket suspension will usually have a rougher ride than stock. No companies that I know of are making kits for more ride comfort or durability. You can get a strut tower brace and maybe a K-member brace to stiffen up the chassis a little, Steeda makes one to stiffen the rear by the trunk opening.