Hi Radrog,
I admire your ambition. Most of the topics you should be concerned about have been identified already. I presume you have the knowledge and tools to build the cabinet.
Building properly sized (tuned) cabinets and avoiding interior standing wave frequencies takes some knowledge and experience. Building a crossover that hands the frequencys off to the proper drivers, controls excess peak energy at specific frequencys, and manages the ohm load placed on the amplifier, is also a skill requiring a great deal of knowledge and experience.
Tools for this element would include various books, crossover and box software, and output measuring/graphing software.
If you wish to proceed with the learning experience, I would recommend diving in and enjoying it. You will undoubtedly experience many trial and errors as you refine your speakers. If you want the enjoyment of building it yourself, with reasonable chance of success, and cost savings, I would recommend this path:
Complete kit with drivers and crossover, and cabinet plans: http://www.selahaudio.com/id141.html
If you want to do it all with plans for the complete project: http://www.partsexpress.com/projectshowcase/projects.cfm
Here is a site from Seas drivers, with recommended DIY projects: http://www.seas.no/index.php?option=com_content&task=view&id=28&Itemid=38 This project is available from the Madisonsound catalog.
Scanspeak may provide similar help.
These options would put you in charge of building the quality cabinet, and reduce the amount of engineering you would have to do for the crossover and cabinet design elements.
Good luck,
Mark