hmmm, interesting. I'm with most everyone else on here. Verticle dropouts and single speed can be tough. Of course, you can fix anything with money. And then you plan on swapping out cranks and adding a cassette for sometimes? Me thinks, n+1 for you. Although you say that you have time and knowledge to do it, it'll become a hassle IMO. And building a bike can be complicated. That's why you need to build a full plan. And there are LOTS of parts to choose from. And then when you start mixing stuff up, things get really interesting. I would never do a fixed gear where I live. I have too many hills to contend with and in some places in the city, it's always windy.

Good luck though!