Well, you really have two options. One is you that you base it on a starbase but give it custom upgrades and stats so it fills the role you desire. The other is to base it on a turret and have the stats change through research. Both can be made to work, but the real difference is that with the starbase approach each and every platform you build will have to be manually upgraded, while the research based approach will affect all of your platforms with one upgrade.

If by skin you mean what mesh/texture, its really a matter of taste. Any structure or starbase can theoretically work, though you may have to resize the mesh (there is a tool for that) and adjust the weapon points to get it to look convincing.