The build part is easier than it might first seem - I am doing it now. Once you have a grasp of these machines fit together, it is pretty straightforward. In my case, I didn't need a special tool for the headset, although I have acquired a few tools along the way. I bought a frame off ebay, and then worked with my LBS to buy the componentry from them. I could get it cheaper through the internet, but I wanted to 1) support my LBS, and 2) have someone who could help if I screwed up. Although I haven't finished, I got a good deal on the frame and a not-so-bad deal on the components, so it should come up fairly close to retail when I'm done. I was going to get a rusty clunker to start, but I wanted something nice when I was done, so I ended up with a higher-end bike than I first started out thinking about. But it's not a problem
The more interesting question will be the fit. There are a few approaches. You can use the online calculators mentioned above, most of which are pretty good, but use several since there a few that give (IMO) bizarre numbers. You could also go to your LBS as though you are going to buy a bike, and they will try to size you. If you really have no intention of buying a bike, you might consider this a little disingenuous (there are probably plenty of rationalizations - e.g., you will buy service and parts from them , etc.). Or you can find a place which offers the service of fitting you (the service that you pay for is the fitting itself). I found a number of places that would do this for me, although it is typically not too cheap if you go to someone who's good. One variation on this is Serotta fitters who do a fitting and then use an "X-Y tool" to compare this with stock bikes. Or, you can combine all these approaches; I'll bet you will learn about fit in a fairly short time.
Good luck and let us know how you make out.