If you have a few parts already, and have the frame, all you have to do is fill in the rest of the components to get your bike. The actual act of putting it all together isn't all that hard as long as you follow some directions and use appropriate torque/grease. Don't cut your brake cables/housing too short and make sure you get the right/correct headset and bottom bracket size. You might wind up buying several different sizes of stems and maybe a different handlebar shape over the course of the build...these things happen. You can't really tell exactly what you need until you build the bike up, get on it and ride. I went through two stems and two saddles on my last build to get the right length and comfort right.
I bought a complete component group for my build, and that is usually the way I like to do it. It gives you most of the parts you will need and you know they are going to work right together. Get the cassette and the chain rings that are right for the terrain you are going to ride in. Mine was the standard 50/34 and 11x28 and it seems to work pretty well for me.
I usually "build" my bikes from parts because it affords me the ability to buy and hide parts/frame from my wife over the course of time. She keeps me on a tight money leash, so....I do what I can. I also get the bike with the parts I want instead of purchasing a complete bike and then go about replacing parts I don't want.
But, honestly...if I were going to buy a new 2nd road bike today I would probably go to the local bike shop, a trek dealer, and buy a complete mid range Domane or Emonda and swap my current wheels with the stock ones. Those are pretty good bikes with good parts spec and it will come in somewhat cheaper with less hassle.