To add to the above excellent recommendations, I advise getting familiar with some of the available spoke calculators. I like
spokechart or
spokecalc. Exploring these resources will orient you to all of the measurements that go into determining proper spoke length. Practice using the numbers for your current hub and rim and then do the same for your new hub. If you are adventuresome, you can remove one of your existing spokes and measure it to see if your calculation exercise was accurate.
It does sound from your posts that you have quite a bit of angst about this project. If you can control your fears, plan ahead, and work methodically, you will learn a great deal and will gain confidence in your skills. There is a lot that goes into a properly built wheel but most of the tricky stuff is related to tensioning and truing. Lacing up a wheel is pretty easy (though I have to admit even though I have built hundreds of wheels I still occasionally have to take a laced wheel back apart because I wasn't paying sufficient attention). Once laced you can take it to a shop for the tensioning and truing.