i think the best way to start is to convert a decent old frame with decent parts. unfortunately i've definitely worked with or advised a bunch of people who've heard that the best way to get fixed is to convert an old road bike and they buy some heavy gaspipe monstrosity with gross cranks where you can't seperate the chainrings, and steel rims and all this terrible **** that's completely not worth it. on the flip side i made a conversion for a friend out of a really lovely trek, with fastback seatstays and some decent parts on it, jbwelded a suicide hub and told her to get a track wheel if she ever wanted to wean herself off of handbrakes. and it's a nice bike right now. real nice.