I was in a similar position as you a few years ago thinking about getting into randonneuring. Most people I talked to advised me to start with a relatively inexpensive bike and complete a few series first before going with something custom. The logic is that you don’t know what you like until you have tried it a few times.
I ended up finding a new previous model year Jamis Aurora Elite at a local shop for around $1200. All I had to do was add a trunk bag, speedo, a couple of lights, and I was in business. I just completed my first full series this year on that bike and have decided that at least for the foreseeable future, a 1200k is not realistic given my family commitments. Since I’ll just be attempting one series a year for the next several years, I decided that the bike I have now is totally adequate for that. Even the battery powered lights are fine.
You may consider a similar approach, maybe ride a full series in 2017 & 18 and throw in a long one or two on a starter bike, then get a new bike for the PBP season in 2019 if you still have that goal. Just another option to think about. As an aside, I met a guy in our club who has a beautiful full-on custom rando 650b with dynamo, GB front bag, the works, and he hasn’t yet been able to fit a full series into his schedule. I’m sure he had the best intentions when he got the bike, but life just got in the way.