The first Bottlehead kit I completed was a SEX amp, followed by the SEX upgrade. After that, Paramounts, a Seduction, a FPIII, and now a BeePre in work. I have an Eros still in my project backlog, and it looks to me to be one of the more involved builds.
If I was to do it from easiest on up, I'd have started with the Seduction, probably followed by the SEX, the Paramounts, and FPIII. However, they all use the same skills, so if you can do one of them, you should be able to do any.
In my experience, trying to an upgrade in the midst of the original build is a great way to get yourself stalled. If you do choose to do that, keep it really simple - capacitors might be the sort of thing that wouldn't be too problematic.