Welcome to the forums!
My first build was a seven, and I was a noob then. So I say go for it!
I also started by buying the Hiscock "bible" - believe the hype!!!
I posted this somewhere before, but....
re Hiscock's book:
1. Read it. All of it. Especially the parts you don't understand.
2. Read it again. Especially the parts you don't understand.
3. Ask relevant questions on here about the parts you don't understand.
4. See 1.
5. Consider buying a piece of wood.
....or something like that, I'm sure you get the idea by now!!
I too would advise you to draw your own plans. There are several advantages:
Experience of drawing plans - helps organise the mind and focus on details.
You can accomodate your own styles and tastes into your design.
Make your plan into templates.
Good advice from dpm99 there about not buying stuff until you need it!
Specifically regarding a seven string as opposed to a six-er, here are some things to consider:
Hardware you'll need:
7 string bridge, pickups, nut - all readily available, but needed for measurements for routs etc.
a 7th tuner (obvious I know, but they mostly come in sets of 6, so ask around and some sellers will make you up a set of seven)
ferrules, if you're using them! (Again, normally supplied in sets of six)
(My advice: buy all the hardware early, you'll need it handy for measuring at various points during the build. Many things are decided by other factors, such as heights and placements, so it helps to have as much of the bits to hand as possible!)
Also think about:
wider neck pocket necessary (if it's a bolt-on neck)
headstock design will have to accomodate a 7th tuner, and still look right!
Hope some of this helps!!
DJ