You don't need one, but they are nice to have if you get one and you're very particular about what you want. I had to really learn a lot about options just to order mine and get dialed in exactly what I wanted. It turned out beautifully, but again there was work on my end in deciding how it was made, from angles and lengths to material choices to paint job, plus what components I wanted to put on it. I didn't go for super lightweight like what you want, but what I had in mind wasn't something you could get out of an off-the-shelf bike, really, so in that sense it was similar to your problem.

The result: My Mercian with Rohloff custom setup: