Prebuild planning is part of the fun for me, enjoy the process!!! It's like window shopping for my wife; I get all giggly and stuffs.
I'll add this to the mix, as has been largely described by LSC and others here (guess I ain't addin' much then, am I?!?); if you have that itch in you to tinker, improve, tinker, improve; build it from the bottom up. If you can honestly admit right now that whatever engine you "complete" first will not be enough; build it from the bottom up.
Building it correctly won't be hard, and experiencing what is layed down here firsthand will bring sooo much clarity; but the biggest benifit will be, IMHO, the courage in tackling harder portions as they may come. Porting, changing tming with a degree wheel, some of the more permanent changes; they will all be more comfortable as you gain that mechanical experience with your engine.