Sam. I would strongly recommend making the visit to homwtheatershack. There are lots of very experienced DIYers that can help you sort through all the design stuff and there is a great program, winisd, for modeling drivers.
There are a couple of guys on the forum who will help you with the modeling. All you need it the T/S parameters of the drivers you are considering. this will allow you to compare the performance of drivers given your restrictions.
Here is an example of the type of resource the shack is. Its a build thread for an 18" Mal-x with passive radiators: massive output in a reasonably sized package.
