An answer (to most of) your questions about building your own seven string can be found here:
(For those who are interested: the site will be updated in the near future with some new photo's and a section about the Lo Pro edge for 7 string.)
Since it's your first I would suggest not to go to complicated in the body design. Start by making something straightforward to learn the basics. Keep it simple. (your first anyway)
It's more to it than just a spectacular design. You also have to play 'that thing' when it's finished. read: "playability". There are many people who have tried to produce revolutionary guitars in the past, and most of these have quickly fallen by the wayside..
The attitude of making a revolutionary design is fine if you have some experiende already, but it can lead to problems if you just jump in that deep end.
As for wood, each piece of wood is unique. The sonic properties of wood vary dramatically between species, weight and density. Choose the appearance/species that best suits your personal taste.
For a description of tone-woods: check Warmoth.com
My advice to you: Inform yourself about guitarmaking. Buy books, search the internet and try to know what you're about to tackle. Only then you will succeed.
I wish you all the best with your upcoming project